Hello Tia and Welcome to forum,
Do you by any chance have any other antivirus tool installed along with avast! ? If you don't, but you had before, which one was it, and are you sure you completely uninstalled it prior to installing avast! antivirus. Having two or more antiviruses with resident shield running, on the same computer, is one of the best ways to screw up your system...
Please try to answer all those questions, so we can try to find a solution for your problem.
Cheers !