Oops, this should never happen.
What level did you use when scanning (Quick/Normal/Thorough)?
Was archive scanning turned on or off? If on, does the problem happen even if you turn it off?
Also, it would be very helpful if you turned on the generation of the report file. To do this, go to program settings (e.g. by right-clicking anywhere in the window), and turn it on (on the Reports page). Tell avast to generate report for all kinds of scanned files (infected files, OK files etc.).
On next scan, this will generate a file with a list of all files avast touched. When it freezes again, take a look at the end of the file and note the last file that's there. It's most likely that the file causing the problems is the one that is located immediately beyond this file (on the disk) - at least you might guess the directory where it's located from it...
Anwyay, do you have any huge files on your disk (>2GB)?
Thanks,
Vlk