Alot of people have trouble booting in Windows Vista and some people might have this:

This is because the Boot Manager is missing and all you need to do is
recover it. Unfortunately, this requires Windows Vista installation
disc. If you do not have one, I suggest you purchase in retails or ask
your computer manufacture.
Insert in the CD and it should ask
you if you want to Boot the CD. If it does not show up, I'm supposing
you should head to the BIOS and change the boot order.
[Image cannot display, because of image size. Sorry!]
Select your language and click next when your ready. If the CD boot fails, reboot again.
You should see a link on the lower left-hand side for "Repair Your Computer"

Alternately, you would attempt to find Vista Installation Directory

The process will attempt to recover the Boot Manager. It might take
several minutes depending on your system and RAM. Startup Repair will
not make changes to personal data and files. So don't worry. It should
work after it's done, reboot your PC and it should be recovered! If
not, retry again or post it in "Help" Section.

It should work and it needs to restart the PC to take effects. Remember to change the boot order before booting in!