To further investigate the issue you may do the following.
1.go to control panel --> administrative tools then select event viewer. check on system logs. you might be able to find a clue on why the notebook keeps on freezing after 15 to 20 minutes.
2.please check your virus definitions if its fully updated. If not please update it then do a full scan afterwards.
3. do a windows update on your system if it's not fully updated yet.
4. You may also check or try to remember what has been installed lately. It might be that the application installed have caused vista to freeze.
Or you may also check this
link for another user who has the same issue.
hope this helps . . .
hope this helps . . .