if you want to play with peer to peer you need.
a) bitcomet (bittorrent client)
b) kazaalite (no idea what its called, I used to use but too much hassle)
c) peer guardian (stops people like mpaa and riaa snooping your pc)
d) spybot defender search & destroy (anti spyware)
e) ad aware ( I don't use it myself though)
f) firewall (zonealarm will do)
g) antivirus (norton or mcafee)
h) NAT firewall (helps quite a bit), its hardware based, if you're using a router of any sort, you most likely have it.