Leo says that Avast may be giving Bob a false positive, which isn't unlikely, especially with free AVS software. If he wants a free antivirus, he should dump Avast and go with Microsoft Security Essentials. He could also download Microsoft Windows Defender. He can even make a bootable USB key or CD and then run it independently to scan his entire hard drive.
He can get a second opinion by going to Eset.com and running their free online virus scan. He also should check out Microsoft's Malicious Software Removal Tool that's already on his computer. To get to it, click "Start" then "Run", type "MRT" and hit enter.
(Disclaimer: ESET is a sponsor).