If it helps anyone, "Microsoft Security Essentials" was rated as the best performing, in terms of low resource usage and detection rates, according to Lifehacker (a blog). See
HERE.
Of course there is a lot of debate over which anti-virus program is the best, and almost everyone has an opinion.
Something you should keep in mind - My current PC is very powerful, and it handles MSSE with no issues. On an older P4, I did notice that MSSE bogged down the processor for about a day after it was installed (did not get turned off for the duration). After that period of time, MSSE's processor usage dropped. I'm guessing that MSSE was analyzing files in the background after it was installed.
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