AVG by girsoft is free and works well I have used it for 4 yrs no prob and has an auto update.
http://free.grisoft.com/doc/1
download page
http://free.grisoft.com/doc/2/lng/us/tpl/v5
bott of page file name AVG 71 free 385a729.exe
Adaware is really good to remove advertising guff and the like nuisance remover ,stuff not considered viral
http://www.lavasoft.de/software/adaware/
Longclaw
