The Windows Vista Recovery Environment can be useful in restoring system files affecting startup.
To use this feature:
1. Boot from the Vista Boot DVD
2. scroll through to choose a recovery tool
3. Statup repair can replace or repair corrupted system files. This is the first option shown
If startup repair is not able to resolve the issue and allow you to boot into Windows you will see the a dialogue window explaining that the repair was not successful along with problem details.
At this point you should contact your IT Admin or call tech support.