I need some info and advice, please.

I've been having problems lately with viruses and Norton has stopped getting its updates (presumably from a virus that tells it 'done, you've been updated' when really it didn't even connect at all). I've downloaded Kazaa's Bullguard, and it detected at least one trojan. I do download a lot of stuff from Kazaa, Limewire, Winmx, etc., so, granted, I've been playing with fire, but so be it.

My question's are: which P2P/fileshare networks are the most notorious for viruses and hackers? Are Norton and McAfee etc., really effective, or are the hackers always one step ahead?

I've also had a couple instances lately where I'll be using a program (a game, Explorer, WinAmp, whatever) and suddenly it'll disappear as though it was never running at all. No freeze or stutter, no time while the computer is busy with something, no error message, no nothing, just BAM, desktop is staring at you. Weird. Anybody know what that's about?

I know these types of questions are beaten to death on a website like this, but any and all info or advice would be really welcome.

Thanks.