How exactly does it freeze? Does it happen always approximately at the same place/folder of the scan?
Do you see a hard disk activity (when it freezes)? Is only the program frozen, or the whole computer (possibly with mouse cursor)?
You can go to the program settings and turn on the creation of the report file (with "OK files" to be included as well in the report). This way, you can find out where the scan really stopped (it's going to be close to the end of the report).
After avast! disappears, check the end of the report file - the "troublesome" file is likely to be close to the end (close in the sense that this particular file will probably not be written in the report, but the previous one will be the last line, so it shouldn't be hard to guess). They would certainly like to have this file - if it really causes problems to avast! - so that they could fix the problem.
The report file created (if you turned it on in program settings) will be (default location) at <avast4>\Data\Report\Simple User Interface.txt
Also, let's say clearly: if during an avast! scan,
1. avast! freezes or crashes, it's probably avast! problem.
2. The computer freezes or restarts, it could be e.g. a conflict with some other program, or a hardware problem.
3. The computer powers down - it's probably a hardware problem (e.g. overheating). The scan generates a lot of hard disk activity (and is also CPU intensive), which may increase the temperature of your hardware. I suggest to check the coolers in the case.
4. If you get a blue screen, it could have a bug in avast! drivers (but then, there is not a power down).
