It didn't reboot on my XP pro but it didn't update the rev number from 549 either. I had to stop Avast from the On Access Control panel & restart it to get the correct rev number in "About". Not a major drama, just not clean.
Well, it does not ask for a reboot but to change the version number you will need to do it manually or, what is the most common thing, see it on next reboot.
BTW..Vlk, the dup "Clean" message problem has disappeared for the moment. It was coming & going all the time. Maybe another Y2K5 prob!! 
About the clean notes into the message body, can you try 'Settings' on my signature?
UseAlternateRtfSyncing value under [MailScanner] section.
See screenshot of settings here:
http://forum.avast.com/index.php?board=2;action=display;threadid=9514
