I'm a certified tech and have a good deal of knowledge of antivirus and firewalls. I've found to insytall avast home (a free Program found at Free antivirus - avast! 4 Home Edition Download ) It has a pre-boot scan option that scans your computer before any thing is loaded into memory. I have found this program Is all I've used. Zonealarm used to provide a free firewall also but I don't think it's available for free anymore. Anyway Between this and the online scans is the simplest and easiest way to protect and service your pc. Be sure to save the set-up package and then install so you can always uninstall and reinstall when you run into this problem.