Welcome
These instructions, may get you to boot and then solve the problem

If you create a recovery DVD, you may be able to boot to recovery options
Create Recovery DVD.
Hard Disc Controller must be set to IDE in BIOS and Bios must be set to boot from the CD Boot Priority - Change

The Vista recovery disk is used to boot Vista when it won't boot - to run Startup Repair, System Restore, access a Command Line, Recover using a Vista Backup Image, etc.
System Recovery Options. Choose an option tht will not cause you to lose your stuff.

If Vista will not start now, boot the Recovery CD to run Startup Repair which automates System Restore as well as numerous other tests including System File Checker which can repair system files which are not corrupted beyond repair.

If Vista still will not start, try booting into Safe Mode to run sfc /scannow to repair System files.
Special Thanks Given to Seven Forums.





Below is a bulletin distributed by MS to explain the start up procedure when Windows will not boot.

• Startup Repair. Startup Repair is a Windows recovery tool that can fix certain problems, such as missing or damaged system files, that might prevent Windows from starting. Startup Repair is located on the System Recovery Options menu, which is on the Windows Vista installation disc. If your computer has preinstalled recovery options, it might also be stored on your computer's hard disk. For more information, see Startup Repair: frequently asked questions and What are the system recovery options in Windows Vista? If your computer does not include Startup Repair, your computer manufacturer might have customized or replaced the tool. Check the information that came with your computer or go to the manufacturer's website.
• Change restart settings in safe mode. If your computer is stuck in a loop where Windows fails, attempts to restart, and then fails again repeatedly, restart your computer in safe mode and select the Disable automatic restart on system failure checkbox. For more information, see Advanced startup options (including safe mode).
• Reinstall. If no other options have solved the problem, and you are sure that your computer's hardware is functioning normally, you might need to reinstall Windows. A custom (clean) installation of Windows will permanently delete all of the files on your computer and reinstall Windows, so only use this option if all other recovery options have been unsuccessful. After the installation, you will need to reinstall your programs and restore your files from backup copies. For more information, see Installing and reinstalling Windows**.

What is the make and model of your computer?

Follow these instructions to go back to the day you bought your computer. Your stuff and everything that you did will be gone
Recover Windows 7 Operating System Using HP Recovery HP Pavilion dv6646us Entertainment Notebook PC - HP Customer Care (United States - English)
Same for Vista.