Could be allot of different things type of file Mp3/.exe/wmv. It could be a corrupt file. Since you are a newbie I would suggest you read some of the sticky's provided in the forum I will give you a couple of links to some very important ones the rest is up to you.
http://www.gnutellaforums.com/showth...threadid=41432 http://www.gnutellaforums.com/showth...threadid=35986
I would suggest you read through these. The first link is about Viruses on the network which from reading your post there is a good chance you may have already downloaded one and opened it. Thus infecting your computer.