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If you use the previous stable version, and you were not having problems with it, then you can reboot any time you want.
Now, Avast 6.0.1203 was having some problems under XP, and some patches were available. Those patches were included in the new 6.0.1289. If you still want to use the previous stale Free version, filehippo.com has them (but, to be clear, I’m NOT recommending it; better to try to solve the current problem).
To clear any old remnants that could be leading to the BSOD, you might want to use Avast Clear Uninstall Utility under Windows Safe Mode, from http://www.avast.com/uninstall-utility, and then reinstall the latest 6.0.1289 under Normal Mode.