Well, I guess you should first examine the scan log to see where avast is starting to hang.
The only problem is that I don't remember the name of the logfile, but I know where it's supposed to be. If you can, look around and see if you can find it, or wait until someone else can tell you what the filename is.
It will be located here: C:\Program Files\Alwil Software\Avast4\DATA\log
Also, by scheduling a boot time scan, it may allow the scan to complete.