HouseCall: On-Line Virus Checker
Trendmicro is the maker of the best virus checker, PC-Cillion.
They offer a FREE, FAST, EFFECTIVE, and NON-INVASIVE on-line version. Better safe than sorry! Check now, and check often!
(NOTE: For many reasons, we highly recommend PC-Cillion Anti-Virus software over McAfee or Norton)

BrowserTune.com
You are just minutes away from knowing if your browser's most essential functions are working properly, and if your Internet connection is up to speed!  You are also minutes away from being able to zero in on any of over 300 browser features and functions to see exactly what your browser supports (or doesn't!). Nothing to download or install!

PC Pitstop
At PC Pitstop they can help you get your PC in top form -- running fast, stable and secure. More useful tools than you can imagine!  PC Pitstop runs diagnostics on your PC to identify things that might help improve performance. The process is fully automated, private and safe. After the diagnostics run, They'll give you tips for improving all kinds of things. Best of all, the service is free!

Disabling Those Annoying MS Messenger Popups!
This is a feature of the Windows Network, NOT an actual bug or true security leak. However, it IS being exploited by spammers. Disabling this feature of the operating system may break programs that rely on internal networking, however, most home users will be unaffected. Do this at your own risk. Keep in mind that running virus protection on your computer WITH a Firewall is the BEST way to protect your system.