There are trusted sources for mp3 downloads?

Also I really /really/ wouldn't recommend running more than one anti-virus program at a time.

For full anti-spyware/adware/virii support I'd reccommend just sticking with an all in one product like Norton 360.
Norton had a really bad reputation for being a bit of a resource-hog but the newer versions are much better.

Also, obligatory piracy kills the music industry etc etc, but if you insist on pirating music, don't use limewire/"free mp3 download" sites/rapidshare. There are... at least two other and just as popular ways of doing it that I can think of that are far /far/ safer in terms of malware.

Or you could be completely safe and cough up the $1.69AUD to buy a song off the iTunes Store.