I mentioned Microsoft Safety Scanner a while ago, but having recently needed to use it in anger I reckon it's worth another plug.
Whether or not you have up-to-date antivirus and anti-malware programs on your PC, there will always be the occasional moment when you suspect that something's not quite right and you want a second opinion. Installing more than one standard antivirus program on your computer at the same time isn't a good idea, so what can be done?
The answer, or at least one very good answer, is to download Microsoft Security Scanner. It scans your PC for viruses and malware. It's free. It doesn't need installing - just download the .EXE file and run it. It doesn't need updating, either, because your download is only valid for 10 days, after which you'll need to download a new updated version.
The program is simple to use, works on Windows XP and above, and is definitely worth keeping in mind if you ever experience weird behaviour on your PC, such as unexpected slowdowns or perhaps some strange popups. The only downside is that the download runs to 70 MB, but that's a small price to pay for peace of mind.
You can get Safety Scanner from http://www.microsoft.com/security/scanner/en-us/default.aspx . Just follow the link and click the big Download button at the top of the page.