Before opening the box, shut down and switch off all power supplies. Look at the leads coming out of the back, and make sure you know which goes where, as you will have to put them all back again in the right places.

• Draw a diagram of the back, and write down what connects to which socket. Note unused sockets, too, so you don't spend time wondering about them later.
• Label the power plugs. It is likely you have at least two which include transformers, which look alike but do not deliver the same output current to their devices (External Modem, Speakers etc). Plugging the wrong ones back in will blow something!
• Label the connecting leads. Use or cut up some sticky labels, long enough to write what they are ('Keyboard', 'Mouse', 'Monitor' etc), and then wrap around individual leads.

Now unplug everything except for the mains power lead to the box itself. This should be left in to provide a ground (earth) wire to conduct away any static electrical charges created by you.

You can purchase anti-static wrist straps if this makes you feel less nervous. Alternatively, wrap the ends of a standard two-ply electric wire with the insulation sheath stripped back just enough to link round your wrists at one end and a bare, unpainted part of a radiator or water pipe at the other - not the mains. If you are prone to static shocks this should definitely be done. If you use one of these methods, unplug the mains lead to the machine, too.

If your user manual has diagrams or pictures of the internal layout, have it to hand. You may as well try to discover roughly what's what while the case is open.

Depending upon design, the box will either have a number of screws which hold the cover panel(s) to the frame, or catches to hold sliding panels in place. Undo the screws or release the catches, and remove the sides/top that are meant to be removable as applicable to your machine. Use a screwdriver that fits, and place all the screws to one side so they don't get lost. Only unscrew the ones which definitely hold the side/top panels - you are not dismantling the entire thing! With the access panel(s) off, have a look at what's in there and try to identify the main items. Don't touch them yet, however, and be careful with that screwdriver ... Look for:

1. The Chassis. That's the metal cage onto which everything is attached. Touch this regularly to ensure any static in your hands is conducted safely away to earth.
2. The Battery. It's usually a smallish, round, silver object held in with a clip. This keeps the system clock going and provides enough power to keep the BIOS settings in memory when the mains power is off. When it dies, you need to know where it is, and how to change it, so have a go now, and put it back the same way that it was.
3. The Motherboard. This is the large brown rectangular circuit board into which are plugged lots of other wires and ribbon cables (that's the tangle of wide, grey wires), plus electronic objects you don't need to worry about. It also hold the RAM chips.
4. RAM chips. The Memory chips are thinnish black things plugged into long sockets on the motherboard. You should know where they are, if only to know what to avoid touching most!
5. The CPU. The Central Processing Unit is what does most of the work. It may be square and sit flat on the motherboard or be a flat box standing up from the board. It will have a small fan.
6. The Power Supply. This will be where the mains lead plugs in, and also has a fan. It may be enclosed in a separate case.
7. Floppy and CD-ROM Drives. These should be obvious as they will line up with their external doors.
8. The Hard Drive. This may be less obvious but is usually mounted in the same area as the floppy drive, and is simply a flat metal box with a ribbon cable attached. Yes, it really is that small, despite having Gigabytes of space!
9. Expansion Cards. These are smaller circuit boards which plug into the motherboard and are fixed in place with single screws, often the same size of screw as the casing screws. On the outside they have sockets for the leads you have removed to the monitor, speakers, and so on. The most likely cards you will have are:
   • Graphics card, to drive the monitor display.
   • Sound card, to drive the speakers.
   • Internal Modem.
   Some leads (eg to the mouse, keyboard and printer) plug directly into sockets attached to the motherboard.

This is where the Hair Dryer, Compressed Air and Electronic Cleaning Solvent (or Tri-chlorethane) come in. Use the Hair Dryer first (set to 'cold' air) to blow dust and general bits of dirt out of the case. Keep the dryer itself away from the RAM chips to avoid the electro-magnetic effects of the motor affecting them. In most cases this should be sufficient.

If there is more debris left, use the can (with the tube on the nozzle) only to blast really stubbborn dust and dirt out. If used before the hair dryer, it will act as a 'sand-blaster'! Use short bursts, and keep the end of the tube away from electrical contacts, or things that look as if they may be. This is to prevent the compressed air condensing and wetting such parts. Ensure the fans are given a spray to dislodge anything which may be blocking them.

Use the Solvent sparingly, to clean the insides of the casing only.

Do not poke about inside the power supply, whose capacitors will remain charged and potentially lethal for some time after the power supply has been turned off.

For a more thorough cleanup - especially if there is evidence of a build up of smoke stains or some devices don't always connect properly - you will need to disconnect cables. Use the same method to identify which belongs where as you used to unplug the external leads, and pay particular attention to which way round the ribbon cables go. There will be a number of spare plugs; ignore them, or use a coloured marker pen to indicate they don't plug into anything. You can also use a marker to label the ribbon leads, taking notes (eg: A=Hard Drive, B=Floppy etc). Any labels will have to be removed before you shut the box, as you don't want them drying out and falling off, but make sure they stick before unplugging them. You may find it easier to unplug one or two at a time, rather than the whole lot. They should be firmly in place, so ease them out pulling as hard as necessary, but avoid twisting as this can damage the pins, and do not be tempted to use pliers or a screwdriver.

Expansion cards (modem, soundcard etc) will also require more force than you might expect. Note which slots in where (draw a diagram), unscrew the single retaining screws, grip them by the edges only and ease them out.

After carefully re-connecting everything, blow any further debris out with the dryer, but don't replace the cover panel(s) yet. Re-connect external leads, cleaning them as you do so. Turn the mains power back on, power up, try all your hardware (CD-player, Floppy drive, Printer, Modem) and watch for signs of problems; the most likely cause will be a cable or card not quite pushed home.

While the box is open, check that the fans are running. If one isn't, now is the time to investigate before overheating damages anything. Assuming all appears to be OK, shut down and turn off the power; replace the casing panel(s).

Note: This is not the same procedure for Pentium IIs and above, or equivalents, where the fan is built into the unit. If your CPU looks like a small cartridge or a tape cassette with mounting arms, refer to the User Manual before attempting removal. If you decide to go ahead, you will probably have to use similar force to unplug it from its slot as you used on the sound card etc. Ensure you have released the retaining clips first, if so instructed.

If the CPU Fan is not working, or is distinctly grubby, it can easily be removed in the case of older ones, and the heatsink also given a clean up. You will need a smaller screwdriver. Unscrew the fan mount screws. Lift the fan unit off the heatsink (a metal grid on top of the CPU itself, designed to disperse the considerable heat it generates when computing). Unplug the fan power lead (two small wires) at the end away from the fan unit; note where and which way round the small plug should fit back.

Use the Solvent on a cotton bud to give the blades a more thorough clean on both sides, the inside of the mount and the mount itself. Then give the heatsink a blast of air, and use the Solvent to clean off any grime. (It should not be necessary to remove the CPU / heatsink unit from the motherboard). Replace the fan, ensuring it is centrally placed on the heatsink. Do not over-tighten the mount screws. They only need to grip the fairly thin metal of the heatsink, which can easily be bent.

Check that it is running before replacing the case panel(s).