Run Windows Update.

Windows Update is usually located on the Start Menu under All Programs.  Running this regularly will ensure you keep up to date with the latest security updates and patches.

Keep your anti-virus up to date and activated.

Anti-virus is one of the most important programs on your computer.  If you don't keep it up to date and activated, you leave yourself exposed to a number of malicious programs floating around on the Web, some of which can cause havok to your computer and can steal your personal details.

Surge protect your equipment.

Power surges and power cuts are a major cause of computer problems.  You can protect your computer and other electronic devices from surges by using a surge protected power gang that you then plug all this valuable equipment into.

Remove any unnecessary programs.

Over time, the amount of programs on your computer will increase.  Often you may install a program to the computer which you either didn't realise you were installing, or you no longer use.  Having these programs installed can begin the slow the machine down, so it is always advisable to remove any of these programs.  The best way to do this is to open up the Control Panel (usually found on the Start Menu) and then selecting Add/Remove Programs (Windows XP) or Programs and Features (Windows Vista & 7).  A list will then populate with everything that is installed on the computer.  Simply click on a program you no longer use, and then click Uninstall.  (Be careful with this as sometimes you can remove a program that controls some function of the computer, such as sound.  Only remove programs that you are sure about).