hello people! I'm italian, so forgive my bad english.
i had the same problem since installing Avast 8, but i didn't realized at first, I had some issue on my pc, file system kind.
my problem was that rebooting after some checkdisk, the system didn't load windows, the screen was black with the cursor. the same if I tried to enter the safe mode: black screen with cursor. I tried to go back to the last configuration working and luckily!, that worked! but you cannot do that evry single time you reboot your pc, do you? so, I uninstalled avast 8, using awsclear in safe mode, rebooted and installed avast 7 again, thanks god i had the installer still saved on my pc. for me, it worked. no more black screen
hope it helps