Probably ok
I have it on two systems at home (Athlon 2500+, XP Pro, 768MB ram, mostly games, AOL, and digi photo use). I did turn off the firewall as I have ZoneAlarm Pro on both. Running fine for 2 weeks now.
At work I have tested it without a problem on a developers laptop. We are mostly W2K at work but I haven't tried it on my primary XP box yet, which does some email archiving, and runs our tape backup robot. Although only because of the notice of a possible conflict with Backup Exec. Probably okay, but I need a Saturday sometime to think it through and do it. If you have old legacy programs you should test first. I'm in no rush to slap it on as my systems are constantly patched and up to date.
I used the 260MB administrators install. It took about 1.5 hours to install on a 366MHz laptop. It took 35 minutes each on my home systems. It forces a restore point creation during the install, and I see that it can be removed if necessary.