Used the following 3 on Vista:
Norton 2007 (came with system/trail software) - Worked well, with no issues,
but it slowed the system down noticeably
McAfee (free from ISP) - HORRIBLE, lots of issues, really taxed the system.
Then it started to interfere with things like the sidebar. 1 week and it was
gone!
AVG Free edition - NICE, best of the 3 by far and no noticeable effects on
any other areas of the system. E-mail working much quicker too and I like the
little certification it adds at the bottom of the e-mails and RSS feeds in
Outlook. A bit ugly in the interface, but simple to figure out. I might
even shell out the bucks and get the Pro edition.