comparison of free virus scanners

I'm thinking of changing virus scanners since I'm about due for an update.
Lately I've been using McAfee on XP, and have used McAfee and Norton on
Win98. I've also used PcCillin on XP/98, and a few of older products on 95.

I'm interested in the free products, support for older machines running
win95, and avoiding the entire licencing issue for upgrades, multiple
machines, etc, but I do want scanning and cleaning, along with updated
signatures as needed. I've bought so many scanners I'd like a break

So I've checked here and done some searching and found the following:

avast
antivir
vcatch
favs

and I read about avg (not free after 03) and bitdefender (no shield),
comments about avast seem positive, but I haven't seen much on the others.

Now the question is how do these products compare? I expect I will try
them all out on a win98 machine and see how they do, but a few comments
from the readers of the group might save me a tonne of wasted time!

/bill