I have BDAV2012, build 15.0.35.1486 running on 2 machines, both XP SP3. Have dealt with the following scenario, on both machines, on several occasions, most recently today. (And also with the previous build level, 1437.)
Following an automatic update, "reboot required". Re-booted successfully. Next auto-update, "reboot required" again. Rebooted again. Next auto-update, same thing - reboot required, etc., continuing like this, over & over again.
I observed, in the BitDefender program folder, there was a large number of files with a "upd" extension (144 of them, to be exact, in today's occurrence). After each reboot they are still there. Now, if I force an update (by clicking "update now", followed by the requested reboot, they are finally gone.
Conclusion: the auto-update is not properly completing the update after the reboot, evidenced by the remaining presence of the *.upd files.
Chris