For the past 6 months or so I've been unable to run a full scan. It gets to about 75-80% and then crashes. I often can't do anything on my computer either and then have to reboot it. It's fine for the quick scan and i don't recall installing anything over this time. I read somewhere that it might be getting stuck on a compressed file but have no idea how to work out if this is the case or if it's a problem with the virus software.
It could be tripping over some zipped files which wont unpack properly or hard drive disk-read errors. For the latter I would try a chkdsk /r - for the former it's difficult to pinpoint in a forum, best to contact Technical Support, it's free by phone or online chat and linked under Useful Links at the top of this page.
It could also be that the software is somehow corrupted but again Technical Support could troubleshoot that.