Be careful with what you download - check file sizes & file extensions. See the threads below for more info.
Virus thread: Typical virus spam file sizes - Make a note of them (click on blue link to see).
* WARNING: Viruses on network you should be aware of ! *

Make sure that you're using a good AV program. Keep it updated & use it to scan all files that you download - before you open them. Which AV are you using? If it's not able to deal well with the viruses that you're finding, the second link above has more links to online scans. Or the post below has links to some good freeware.

http://www.gnutellaforums.com/330543-post11.html