A while ago I ended up formating and re-installing XP, due to ongoing problems with another software. (HP printer, which overwrote the NET framework with its' own earlier version, borking the current one. Anyway...)
I already had some updates, like SP3 saved to disk, and it was a long slow processes updating stuff afterward, with many reboots. After not too long, Avast worked good as gold, but was never much of a problem, anyway. I don't recall disabling it at all for any of the updates, except that I installed SP3 before installing Avast.