I use Avira AntiVir.
If I understand you correctly...you do a full system scan, it finds some items. You eventually get to the screen where items are eliminated or stored in quarantine. Avira then says that it must reboot to remove certain items. System starts to boot...and just hangs.
I've never experienced a shutdown issue with AAV...but I would remove it via Add/Remove Programs...then install the latest version that is downloadable, promptly update same...and scan your system. My guess would be that some file is damaged, which prevents it from properly triggering the reboot.http://www.free-av.com/en/trialpay_downloa..._antivirus.html
. I suggest the Major Geeks website.
Scroll down to Shutdown Does Not Complete Or Hangs, for an idea of what might be happening: http://www.theeldergeek.com/shutdown_issues_in_xp.htm