Best Free Antivirus program?

Is there a good free antivirus program for Windows, or is that an oxymoron?

A friend of mine is looking for antivirus software for her PC, but is
very low on cash at the moment. I tried installing AVG on the
machine, but the program was horrible. The UI was extremely crude, the
autoupdate failed, and just seemed to be all-around junk.

For now I just put a trial version of NOD32 on the machine and it works
quite well. Do any of you have any suggestions as for what to do
when the trial runs out? (aside from purchasing the software)

thanks.
-WD