Can anyone tell me a good and free virus-scanner?

AVG, you'll have to google it. Last time I linked i got in trouble for being too helpful

Malwarebytes have an excellent free scanner. It saved my machine many times. Just Google Malwarebytes Anti Malware free scanner. The free version won't update automatically but you can get updates manually by clicking the update tab. Also have a look at Norton for added protection. I'm sure others on this site will have some goodies too. Oh yeah, do you defragment your drive regularly? That can help a bit too. Also try running chkdsk/f from Run.. in the start menu. That should fix file system problems that can make your machine stutter and hang.

AVG is decent http://free.avg.com/us-en/homepage However there are far superior applications you have to purchase. The question I have is how valuable are you files? Could you stand to lose tax document, children's pictures, email etc.? It may be worth getting a premium product.

AVG is very good. Problem you will have is a virus on a PC will stay on the PC if in the hard drive. Nothing a virus scanner can do. Look at www.ubuntu.com . Linux OS that will take headaches away from microsoft software. No viruses, and is wicked fast.

AVG - Free

I recomend you download FREE "Microsoft Security Essentials" I've used it now for 3 months and find it very good. You can scan any drive you have connected to your laptop or PC. www.microsoft.com/security_essentials Hope this helps

You can get an online scan for free, but if you use a free virus scanner remember that you get what you pay for. If your computer is infected, you can reformat the hard drive for free.

I love Avira. Specially the live CD. Otherwise AVG is good too.