I, myself, have a PC with XP SP3 and I recently installed avast5 on my parents PC, which is also XP SP3. On an initial bootup, both of our computers will lock up shortly after the boot. An immediate reboot solves the problem and all is well again. Only issue I seem to have with avast5. I used the removal tool before installing, also.
I don't know if there is any other info I can give you, any kind of event log screen or anything.
Oh, and yes, we both have .396 installed.