I've always used McAfee VirusScan Enterprise because I can get it from work, free with infinite updates, because of the corporate licence we have. It's not too bad really, you just need to disable the on-access scan while installing stuff from CD otherwise the computer runs really slow. You can select what should happen when a virus is detected, and you can exclude selected files, folders, and file types from the scan. IMO a virus scanner is mandatory otherwise you WILL be infected in a short time after connecting to the internet. When I was rebuilding the hard drive I thought I caught a virus because svchost.exe kept going to 99% CPU, and sometimes ads would appear on-screen. The ads turned out to be messages sent via the the admin messenger port from outside. So I turned off the Messenger service and blocked the port. The svchost problem went away after applying windows updates, and turning off the DHCP service.