hi!
I'm running WinXP Pro w/sp2 on a P4 2.4 ghz w/1 g ram, the mobo is a gigabyte with SCSi chipset (not the best out there).
For firewall I use the built-in one. I tried CPF 3 some days ago and I *really* like it, it's a very major version over earlier 2.4. It launches faster than previous version yet it's way more powerfull - and lighter on resources.
Actually I uninstalled it because I don't need a real-and-robust firewall (I used ZA and Tiny for several years, never can do it with Outpost) and sp2 built-in just works fine, but may be I give it a try again when the Spanish version hits the web.
To achieve maximun performance I have deactivated some services I never use/need, do a hard-disk cleanup followed by a registry clean-up and registry defragmentation (TuneUp Utilities) then I run an off-line followed by full/by access on-line hard disk defrag (PerfectDisk 8). Then, make a partition image with Acronis at boot.
One of the things a like more about avast! is its low impact on system performance and its high detection rate - despite some nasty malware I found the other day and avast! didn't recognize it.
See you!