Tim has a message popping up that asks which app to open a file with. It happens automatically and he doesn't know what file it is. Leo says that's disconcerting. Leo suspects AdWare or worse! There's something on his system that is running in the background and the antivirus can't kill it. He'll have to figure out what the app is that's starting up. He'll have to expect that his system has been compromised, though, and the only real way to be sure he's gotten rid of the malware is to backup his data, wipe his hard drive, and reinstall Windows.
If he doesn't want to do that yet, he should run Autoruns from Microsoft. He can get it from technet.microsoft.com.