This guide will show you basic steps you can take to maintain your computer.

[edit] Steps

Remove all of the remnants that browsers leave over. These includes cookies, history, and cache files. This will also boost your privacy.
Frequently run an anti-virus and anti-spyware program. Try to run these on a schedule of once per week. Also run an anti-rootkit program once every six months.
Remove unnecessary programs at start up. Under windows this can be done under Run > MSConfig.exe. In Linux it can be done under various graphical interfaces such as KDE and Gnome. It will vary upon distribution and interface.
If you run a Windows operating system, it is advised to reinstall Windows on an annual basis. this is unnecessary, however it removes all excess files created by programs which you may no longer use.
Also under Windows, defragment your hard drive every three months. This will reorganize the files in a logical order in your hard drive, so your computer will be able to call the files up faster. Linux uses a different filesystem, which does not require defragmentation.
Physically clean your computer every six months. Use either compressed air or a non-static vacuum to clean the computer. Using a regular vacuum will result in a static build up which can destroy components of the computer.
Turn your computer off during periods in which you do not use your computer. Leaving it on will increase wear and tear on the hard drive, and on the CPU.