Here are a few online scans you can run. Stuff near the top, are the better ones.

Panda Software Active Scan (Great Scan - Make sure you save your log when you scan)

TrendMicro's Online Scan (They've recently changed this around, not so sure I like this)

TrendMirco's Online Scan (Supports non-IE browsers & uses Java) (works well when activex is messing up, or when you use FireFox or Netscape browsers)

Symantec Security Check (Online Virus Scan/Security Check - Will NOT clean your system)
Online Kaspersky Scan (never used it, not sure how good it is)

Online Trojan Scan

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Free Antivirus Program (yes, even I use it)
AVG FREE (Good little Antivirus program. And its free. Can't really beat that. Uses low system resources, doesn't seemt conflict with other software, even does its own definition updates.