On my TR I use Norton Antivirus 2003. It’s fairly unintrusive and I only have it watching my e-mail and scripts. I turned off the auto-protect feature because I’m pretty safe when it comes to downloading stuff and normally do manual scans if necessary. It works extremely well and catches all of the viruses coming into my inbox.
I’ve tried McAfee stuff before and for some reason, it’s so incredibly annoying and has this tendency to slow down any system I install it on. It’s also way too paranoid like the poster mentioned before. I found myself constantly disabling features within McAfee every time I wanted to run certain programs or to test my own VB scripts.
On the security front, I normally only use Windows’ built-in firewall protection since I’m not too paranoid with security for whatever reason.
I also keep a copy of SpyBot 1.2 installed to search for malware/adware products and that works extremely well (and is free!).