It sounds like the "access" permissions in IE have been locked out.
That is why you get the message that you do when you try to open the IE Tools, Intenet Options.
At this point, I would try a System Restore back to a date before you installed Avast.
I am not sure that Avast is the problem, but you need to get access back and the system returned to normal before we can troubleshoot the error.
If your OS does not have the System Restore function, then you must use the DOS function.
At the prompt, enter "scanreg /restore"
This should bring up a simple interface where you select the date to restore the computer to, and click ok.
Be aware that when restoring a system to a prior date, any installations or settings done after that date will be changed.
Good luck
