HouseCall from Trend Micro is a great virus scanner that helps you scan and eliminate viruses from your PC.

Panda Software has a free online tool that scans and eliminates viruses. The paid version also eliminates spyware and removes hacking tools.

Kaspersky File Scanner is great for checking individual files for viruses. If you're curious about a file, get it checked here quickly.

Microsoft Malicious Software Removal Tool scans your PC for some of the most popular and brutal worms crawling the web.

BitDefender Removal Tools. If you suspect your system is compromised you can find specific malware removal tools at the BitDefender website.

No matter which program you use, it's important to keep your system patched and scan frequently for infection. While your PC may not be acting strange, it may be compromised and currently infecting other machines on the Internet without your knowledge.