Sounds like something has happened to your installation of Windows Vista and I suggestion you follow one of the two sets of instructions to repiar your operating system.
1. Boot up with the Vista install disc
2. You should see a screen that says "Windows is loading files"
3. After a few minutes you will get a language option. Select your language and hit next.
4. On the install screen select "Repair your computer"
5. Windows will find your copy of Vista on the machine
6. Select your copy of Vista and click next
7. You will now see the following options.
* Startup repair - This automatically fixes problems that are preventing Windows from starting
* System Restore - This will Restore Windows to an earlier point in time
* Windows Complete PC Restore - This will completely restore your entire PC - programs, system settings and files - from a back up that you have previously created (Available in Windows Vista Business, Enterprise and Ultimate editions only)
* Windows Memory Diagnostic Tool - This checks your computer's memory hardware for errors
* Command Prompt
8. Assuming you are using this option because you were unable to boot to Windows Vista select the system Restore option and then select the date you would like to go back to. If your OS is fine you can always use the startup repair option.
Pre-Installed and OEM versions: (please check with your manufacturer fist. They may have replaced these tools with their own.
1. Turn your computer on and start pressing the F8 key. If you see the Windows Vista Logo you have gone too far and need to reboot and try again
2. You will get an advanced options screen. Select the "Repair your computer option" and hit enter.
3. Select your language and hit next.
4. Enter your user name and password XXXXX then next.
5. The recovery options should now appear.
6. In most cases just using the system restore to set your computer back will solve your issue. If this is not the case try the Startup repair option.