On my family's Windows XP Home SP3, Intel Celeron D, 512 MB system, we had AVG for some time, and I noticed how sluggish it was. The only time that we had malware or viruses was when my little brother installed random toolbars.

That said, I just switched it over to Avira AntiVir. I've heard alot of good things about ESET NOD32 as well.