Hi Elisa31
Operating System??
I just meant which version of Windows.
From your replies, I'd guess it's Windows XP
I think something has corrupted you Windows files so that programs, like avast!, will not run properly.
I'm surprised you aren't getting the same response for any other programs.
I actually had to go back to a previous setting
Did you mean "System Restore"?
If not, XP has feature called System Restore which may save you having to reinstall Windows.
Click "Start" > All Programs > Accessories > System Tools > System Restore > Restore my computer to an earlier time.
Click "Next" and then choose a date in bold, that is earlier than when the problem first started.
Click "Next" again and follow the instructions.
The computer will eventually restart and show a message re success or otherwise of the restoration.