Cilli,
No, this is not a way to get you to reboot. Avast seeks an update, whenever it senses an internet connection. Sometimes, as part of the update, a reboot is necessary. This will normally be required when the "program" is updated.
As for the user profiles, I would recommend that when Avast asks for a reboot, allow it to do so. This way you know that the download and installation has been completed.
Truthfully, I am at a loss to explain why that happens with the profiles.
I have 2 users on my XP machine and have not experienced such behavior.
If not, then when the "user" who was signed on finsihes, have them do the reboot.
The profiles will remain as they were originally.
techie