See "Learn How to Defrag Your Hard Drive" in the Did You Know...? section of Webopedia.

Also see "Tips to Fix a Slow Running Computer" in the Quick Reference section of Webopedia. 

Defrag Guide
How to considerably speed up your defrags.

Get Your PC Ready for Windows Vista
If you plan to upgrade to Vista, there are some things you need to consider first such as which version, data backup and file transfer.

Webopedia Quick Reference: Common Windows PC Issues
Tips to Diagnose and fix a slow running computer.

Webopedia's "Did You Know...?" Section
Use this Webopedia knowledge section for an in-depth overview of specific technologies and occurrences in the areas of Computer Science, The Internet, and Computer Hardware and Software.

Webopedia's Quick Reference Section
Use this Webopedia reference section for information on common Internet and computer facts and occurrences.

Webopedia's Quick Reference: Microsoft Windows Vista
For years, computer users heard of a mysterious new operating system from Microsoft, codenamed Longhorn. In 2005, Microsoft officially removed the Longhorn codename and announced the operating System under the official name of Windows Vista.