Best is to get one of these bootable CD/DVDs with a minimal Linux and a bunch of antivirus progs on that automaticly update over the internet. Since you booted from the optical drive the virus scanners can detect malware sitting in the boot sector of your hard drive. [http://www.avast.com/bart-cd|Here] or [http://www.chip.de/downloads/Ultimate-Boot-CD_13010773.html|here] you'll find working CD images. Try a little google search on the subject and you'll find lots more. Sometimes you find them as giveaways in computer mags.