Okay so this is what I see.
1. Based upon the thread title the Avast is in a scanning loop that hangs the system.
2. Based upon the picture supplied it appears that File System Shield is scanning AvastUI.exe.
3. Based upon the picture supplied it appears that the AvastUI.exe file being scanned is in the Avast 5.x directory path.
So it seemed to me that by doing an uninstall of Avast 5 and maybe even Avast 6 you will be removing the copy of the AvastUI.exe file in the old Avast 5.x directory path that the File System Shield says it is scanning.
Therefore if you remove the file that is being scanned you will remove the loop scan problem.
That is how I see it.
Probably missed something here. What did I miss? Thanks.