Good anti-virus for old computer?

Factual Questions
1

My mother has a fairly old computer - at least 7-9 years old, and I think only 512K memory. It might be even less.

Avast! is bringing it to its knees. Seriously, last time I was over there, it took about 15 minutes to boot, and was slow as a dog even after. Turn off Avast, and voila, problem fixed.

Seeing as she’s not likely to be downloading potentially problematic things, I left it off for now, but I don’t want to leave her without antivirus software forever. Can anyone recommend a small footprint anti-virus package that runs well on older, low-memory computers?

2

Microsoft Security Essentials-firewall, anti-virus and malware protection in one.
Excellent reviews from the computer crowd , too.

I also had an old computer in that age and memory range, dumped Avast! for MSE.