On a PC running Microsoft Windows you reboot by selecting the "Shut Down" option from the Start menu. On Macs, you reboot by selecting the "Restart" option from the Special menu.

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

Troubleshooting System Instability, Reboots and Crashes
Describes what to do if your PC boots successfully but then suddenly reboots itself or hangs for no apparent reason. This page is from "The PC Guide."

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.