Here are a few things to try, in this order :-

1. Go to Start > All Programs > Accessories > System Tools and right-click System Restore and choose a date before you started having problems.

2. Run the System File Checker. This can repair many corrupted system files. Go to Start > All Programs > 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. You can analyse the SFC log file, here http://support.microsoft.com/kb/928228/en-us

3. Copy system files from your Vista DVD that SFC (2 above) identified as corrupt and that it could not repair http://www.vistax64.com/tutorials/86959-access-vista-install-dvd-files.html

4. Run a Vista startup repair. This can repair more than just startup issues http://www.vistax64.com/tutorials/91467-startup-repair.html