Some information about your system might help:
What Operating System are you using ?
Is it a laptop or desktop ?
How big is the hard disk (HDD) and how much data is on it ?
What processor (CPU) does it have and how much RAM ?
What type of scan, e.g. Thorough sensitivity, with or without Archives selected ?
Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
Interesting you say it stops when scanning registry stuff as avast doesn't specifically scan the registry. It scans the files in memory first and then scans the files on your HDD, but not specifically the registry, if during a scan it finds some spyware it will then try to locate any associated registry entries.
So can you give any examples of where it is hanging ?