AVG free is probably your best bet. For even better speed (and only if you're an experienced user) disable on-access scans and use the context menu buttons to scan individual suspicious executables as they come in.
If you do this, beware that viruses can come packaged in image files, sound files, etc, that exploit buffer overflows in popular programs. If you run the latest versions of your programs (incl. Windows Update - Windows is made of many programs) you should be safe enough.
I have mcaffee forced on me by my college network, and it sucks! It's constantly eating up 5-10% of my CPU, and every night it automatically scans, which I have to disable manually. I can't even edit the options due to the other hijacking program they put on that gives the netadmin exclusive control over mcaffee permissions T_T