How exactly does it freeze? Does it happen always approximatelly at the same place/folder of the scan?
Do you see a hard disk activity (when it freezes)? Is only the program frozen, or the whole computer (possibly with mouse cursor)?
Additionally, you can go to the program settings and turn on the creation of the report file (with "OK files" to be included as well in the report). This way, you can find out where the scan really stopped (it's going to be close to the end of the report).
The report file created (if you turned it on in program settings) will be (default location) at <avast4>\Data\Report\Simple User Interface.txt