You're welcome.
1. Virus definition updates shouldn't require a restart.
2. Program updates do require a restart for the changes to be implemented.
That said you still haven't told us what avast program version and build number you are using ?
With XP if you are using avast 18.8.xxxx, this is the last supported avast program version, so you shouldn't be getting any further program updates.
If you have avast version 18.8.xxxx, then you can set program updates to Manual and not Auto.