Used a few since 1997. Mainstay was Norton for several years, beginning with Win98SE.
Norton slowed the system down quite a bit and could be shut down by a malicious file when on the Internet.
Went with CA for a couple, not as good, but then, I got smart, and used Norton Ghost for full disk imaging, so it was much less of a hassle to restore my system to a known clean state before it got infected.
Tried Avast! 4.0 for a short time, was not impressed as it seemed buggy on Win98SE and did not care for the GUI.
First real Avast! was version 5.0, shortly after major upgrade from 4.8. Now used on XP SP3 Home on a Dell 4600i 2.8 P4 2GB RAM along with Secunia PSI v. 2.0.0.3003. There have been about five or six incidents when the Web module intercepted a malicious web page and stopped it cold; no infection.
Currently using Avast! 4.8 on Win98SE dual-boot system.
Have to say Avast! 6.0 is the best so far of all tried; very little impact on system resources. System boot time and performance is almost as quick with the a/v as without.
mchain