Try the video resolution fix: http://www.techsupportforum.com/forums/f59/solved-fallout-3-randomly-freezing-344608.html#post1962376
if that doesn't do the trick, play, let it crash, then immidiatly after crash go to event viewer (after you reboot if necessary) and see if there is any entry in event log detailing the specific module that crashed.
1. Click Start
2. Click “All Programs”.
3. Go to the ffdshow folder.
4. Click “Audio decoder configuration.”
5. In the left hand pane, click “Info & debug.” or "DirectShow control".This is the second entry in the list.
6. In the right hand pane, near the bottom, find a check box labelled “Don’t use ffdshow in:”
7. Check that box, and in the text box underneath (it lights up when you check the check box), add “fallout3.exe” (don’t include the quotes). If there is already something in that box, put a semi-colon ( ; ) after what’s already there, and fallout3.exe after that. Do NOT use spaces, since ffdshow will think that’s part of the filename. A valid entry would be “explorer.exe;fallout3.exe” (without quotes).
8. Finally, click “Ok” and start Fallout 3.
Also do try run as administrator and run in XP service pack 3 compatability mode tricks and see if that has any effect.