AVG Free - Download antivirus and antispyware software for Windows XP and Vista
Free AVG, once its installed, you never have to re-new or re-install it again, and theres never any queries about if you'd like to upgrade or not. Just install it and off you go!
Ive never had any issues with it, it has an update practically everday (which you can turn off if you really want), you can decide if you want it to scan regularly or not, and has a context-menu options when using windoze explorer etc, and its also one of the least system-hogging AV's available!
(I really should get sponsored by AVG, but then again, its free!)