Microsoft's online virus and security scan
You can get a free online Virus and security scan from Windows Live One Care at http://www.microsoft.com/protect/products/computer/safetyscanner.mspx
The scan is specially effective for detecting botnet infections and is recommended as it might be able to pick up what your resident virus or security scanner is not able to, in general its always good to be scanned by another security product than what you have installed.
In addition to checking for viruses and bot infections, it also helps you get rid of junk on your hard drive and optimize your computers overall performance by giving it a free tune up.
Please note that the online scan would not work with the FireFox web browser, you would need to have Internet Explorer. You can get a complete scan done at http://onecare.live.com/site/en-us/default.htm?s_cid=sah/?s_cid=sah