A user updated AVG 7.5 to 8.0 yesterday, and found that it froze her PC on reboot. (After login the desktop background appeared, but no Start button or icons.)
I found the cause was a trial copy of BullGuard which had been supplied with the PC. She had never used it but had not uninstalled it, and it duly expired. AVG 7.5 seemed to be happy to live alongside it, but AVG 8.0 most definitely wasn’t (it’s never a good idea to run two anti-malware products at the same time).
Be sure to remove all other always-running anti-virus products, whether expired or not, before you install AVG 8.0.