Do you see any hard disk activity - after the scan stops progressing? Is any process using any significant amount of CPU at that moment?
Would you be able to create and upload the a dump of avast! service process taken when the scan freezes? Here is how to do that:
1. Disable avast! self-defense (avast! settings / Troubleshooting and uncheck the option "Enable avast! self-defense module")
2. Run the scan and wait for it to freeze
3. Start Windows Task Manager, make sure the option "Show processes from all users" is checked. Select AvastSvc.exe process, rightclick and choose "Create dump file".
4. Wait for the dump generation to complete, go to the folder shown in the window that appears, and upload the file AvastSvc.dmp to
ftp://ftp.avast.com/incoming