I tried a boot time scan not too long ago and it also stalled. I also tried the sfc /scannow process through my command prompt and it showed me that there were some corrupt files that could not be repaired in the form of "iassdo.dll.mui". I have yet to try a safe mode scan, but I am familiar with the process. On on another note, I just updated my Avast and I'm still getting the same problem.