I had a lot of issues with AutoCAD 2009 crashing suddenly. Usually when logging back on when the screen-saver would lock the screen. I worked through the issue with Autodesk and come to find that it was a graphics driver issue. I have a Nvidia FX3700 and was using using the latest "approved" driver from Autodesk. Using the "Tune Log" within the "3dconfig" command in AutoCAD, I got a clue to what AutoCAD was doing with its graphics driver. To see the approved driver list go here:
I actually had to install the third oldest driver on the list to get a stable AutoCAD. The second oldest caused Windows blue screens and required me to boot into safe mode to restore it. I would suggest downloading all of the approved drivers before trying, so that they are available on the computer in case the worse happens.
When we recently upgraded to AutoCAD Mechanical 2009 and a few users could not choose their fits when dimension (ie H7, f6, etc...). It took a while to figure out, but another user found the answer. There was file corruption with the fit data (VAL tables). We copied the files from a known working set (ex. ACADM 2009\Acadm\gen\Val) to the non-working AutoCAD computer (AutoCAD should be shut down while copying the files). The fits are back!