I use NOD32 Antivirus 4, Windows firewall and Spyware Doctor. I've noticed that NOD32 doesn't detect some Trojans or malware.
just Windows firewall, haven't had a single virus or spyware problem in Win7 yet.
I thought the same that there isn't a problem and security is great. If you would check for viruses, you would find them, maybe harmless, but still.