I have used, Avg, Avast and Avira, all excellent, at the time i'd say Avira was superior however the other two have good manual updating for computer without internet connection, avira's did have such a service but it was intermittent due manual update service not being available regularly and was complicated for novice users. Recently spyware terminator free online protection has quiet good according to the youtube videos i have seen, so this coupled with free avast and the like should provide excellent free protection.
Recently im using Kaspersky, which has excellent track record, 2 way firewall that unmatched in my opinion, its an excellent security software with probably no equal freeware alternative however its full scanning is terribly slow to the point of becoming annoying. Avast and Avg were quick on full scan and lighter on resources. Avira scanning is faster than kapsersky as well the last time i used it, which is some while back.