I second ESET NOD32.
I run it on my 566MHz Celeron with 256MB of RAM, and it doesn't cause performance issues at all.
I also run it on a 200MHz Pentium with 96MB of RAM, and no performance impact.
And, a 400MHz Celeron with 64MB of RAM.
It's a wonderful program. Very much worth the price.
I haven't used Norton since.. uh.. 2005?
-John