Hello Andrew,
1. Since when are you facing this issue?
2. Any recent changes made to the computer?
Method 1:
Unplug all the external devices and try booting and check.
Method 2:
Check if you are able to boot in safe mode or not:
http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode
If yes,I suggest you to put the computer inclean boot state and check.
To help troubleshoot error messages and other issues, you can start Windows Vista by using a minimal set of drivers and startup programs. This kind of startup is known as a "clean boot." A clean boot helps eliminate software conflicts.
Refer the link below for more details:
How to troubleshoot a problem by performing a clean boot in Windows Vista:
http://support.microsoft.com/kb/929135
Note:
After you have finished troubleshooting, follow these steps in the above article to reset the computer to start as usual.
If not,try perfomring astartup repair and check:
http://windows.microsoft.com/en-US/windows7/What-are-the-system-recovery-options-in-Windows-7
Method 3:
If not, I suggest you toRebuild the BCD store manually by using the Bcdedit.exe tool.
Refer step 3 to do the same in the below article:
http://support.microsoft.com/kb/2004518
For your reference:
http://social.technet.microsoft.com/Forums/en/itprovistadeployment/thread/4de675a4-14ae-4469-b881-5df58d846bff
Regards,
Savan – Microsoft Support.