My other PC which runs XP (with service pack 3) with Avast Free and Zone Alarm Free has never had a virus except once, when Avast caught it before it had time to anything at all.
It finally died a week ago after 4 years of constant use (hardware, not software, issues). I did switch off automatic updates but I would update manually when it was convenient to me and I always check what the updates are meant to do; eg avoid Windows Genuine Advantage packs. I also never downloaded any new updates until they had been online for a at least a week so that MS had time to receive crash reports from users and repair the update.
The problem with Automatic Updates, whatever time you schedule them, it's always inconvenient and you always have to restart your PC just at the time when you really don't want to.
When I had to re-install Windows XP, I found I needed Service Pack 3 since a number of programs won't install without it (Zone Alarm). Also, keep an older version of Avast handy so that you can install an anti-virus BEFORE you go online.