Many things can cause this issue,the first thing to try is repair the MBR or Master Boot Record.
How to Repair or Fix a broken Vista MBR:
To Restore Vista MBR, please use the following simple solution.
1. Start your computer from the Windows Vista Installation DVD
2. Press a key when prompted to continue
3. Choose your language, time, keyboard and click Next:
Select language and preferences
4. Next, click "Repair your Computer":
Repair your computer
5. Now, from the System Recovery Options dialog, select the "Operating System" you want to repair, then click Next:
System recovery options, choosing your Operating system
6. From the "Choose a Recovery Tool" dialog menu, select "Command Prompt":
Recovery Tool Command Prompt
7. Type the following into the "Command Prompt Window":
8. Remove the Vista Installation DVD and restart your PC.
If all goes well, this process should have enabled you to restore your Vista MBR and you should now be able to boot back into your Windows OS.