1.Clean junk files from your computer. You may use these free system cleaning tools for the purpose.

2.Remove unwanted auto-start entries from Start > Run > msconfig > Startup tab.

3.Use good and resource-light antivirus, antispyware and firewall on your computer to keep your files and other important information safe.

4.Get more RAM and use a good processor. For Windows XP, the preferred requirements are 512MB of RAM and an Intel Pentium IV (or equivalent) processor. For Windows Vista, 1.5GB RAM and any good dual core processor is recommended.

5.Take care to keep at least 15-20% free space on your C:. Remember to defragment your hard drives at least once every month.

6.Keep all your software, including your Windows copy, updated.

7.Format your hard drives and reinstall your operating system once every 1-2 years to bring back the old speed.

:)

[Note]
* For a detailed step-by-step procedure on speeding up your computer, please refer to THIS page.