I installed Avast Free 8.0.1483 on a Dell 8200 Pentium 4 laptop running Windows XP Home SP3 that had never seen Avast before. Windows hangs on the "Windows is shutting down" screen. I tried a Dell 8100 Pentium 3 running the same software, and got the same result. The problem disappears if I uninstall Avast.
I tried both 6.0.1270 and 7.0.1474 and the system shuts down normally. I re-installed 8.0.1483 and the problem returned. As far as I can see, nobody has reported this problem before.
Any suggestions as to how I can identify the cause of the issue? Meanwhile I'll switch to Avast 7. I don't like the Avast 8 interface anyway!