1. Have a look in Event Viewer (Start, All Progs, Admin Tools, Event Viewer) for any clues.
2. You could run the System File Checker:
The command needs to be run from an elevated Command Prompt. Click Start, Accessories and right-click Command Prompt, then ‘Run as Administrator’. Type sfc /scannow (note the space) and press enter. This may take some time and you may have to put the Vista DVD in the drive.
3. Final straw is to repair Vista click here