If you live in an area of extreme electrical unpredictability, this will help you through those unintended power-cut restarts which could result in data loss.
If your country is prone to bad weather a lightning strike can ruin your PC for good in one blow. Try to get a good quality surge protector which also covers your modem or router.
Get a good program such as Norton Anti Virus (www.symantec.com) or the free AVG (www.grisoft.com) and make sure it is kept up to date.
Activate the firewall in Windows XP under Control Panel->Network Connections->Choose your Connection->Advanced Tab (In XP SP2 go to the Security Centre in Control Panel). If the Windows firewall isn't doing its job download the free ZoneAlarm (www.zonealarm.com) or invest in Norton Internet Security and Anti-Virus.
Make sure your PC is fully armed with the critical patches from Microsoft. Check out www.windowsupdate.com to see if you're secure.
Keep your computer away from possible leaks, drips or any situation where water, coca-cola or coffee is likely to spill onto it.
Keep your PC clean inside and out for healthier, more efficient operation. The fans inside your PC attract a lot of dust so once a month turn your PC off, lean the case on its side and look for clips at the bottom. If it doesn't have any, disconnect all cabling and take the side panel off. Then spray the inside with a can of compressed air which you can get from most computer or camera shops. If you are careful you could use a cloth and carefully locate the dusty areas.
Make sure your PC isn't likely to receive any heavy blows, either from smacking into it or from it falling off a desk or shelf.
Just make sure that everybody in your house knows the CD tray isn't a cup holder, not to turn off at the wall and other general tips that may not be obvious to first time users.