What avast is checking is the presence of the symevent device driver. Whit driver collides with avast. As far as we know, it’s part of Norton Antivirus and Norton Internet Security.

If you don’t have such a driver (see HKLM\System\CurrentControlSet\Services\SymEvent), you should have no problem running avast (of course, unless you have some other AV program instead of Norton, such as AVG6 [AVG7 is OK I believe]).

Hope this helps,
Vlk