Will... can only add my

based on what we have been using and after having tested hell knows how many progs. The three below are installed on all boxes (they all have varying functions)
Kaspersky AV - (basic virus dumper) let it update it's virus pattern file daily and schedule to sweep the drives when not using the box.
Lavasoft - (adware dumper) schedule to update daily as above.
RegiGuard - analyses your registry and dumps a load of crap which may be left over from adware and virus progs as above. Run when ya got nothing else to do :-)
All progs above have relatively small footprints and don't screw with other stuff on your box (Norton is a pig for this) and all three should cover 99% of problems. (Never had any problems worth mentioning since the above progs have been installed.)