Do you have a recovery disc which you can boot from the dvd drive and gives you repair options ?
If not, download the .iso file from here ( make sure you get the right one - i.e. 32 bit or 64bit )
Download: Windows Vista x64 Recovery Disc — The NeoSmart Files
and burn it to a cd using this free iso burner. Do not format the cd - if Autoplay pops up, just close it.
Start your pc and watch the black screen with white writing.
One of the first things it says is something like :
F2 for setup
F11 for boot menu (your F numbers may be different )
Press F11 ( or whatever yours is) and a menu will appear. Arrow to CDromdrive, put the cd in, press Enter.
It will ask again, Press any key to boot from cd - Press Enter.
Select a language, a time and currency, and a keyboard or input method, and then click Next.
When the window appears that says Install now in the middle, and Repair my Computer at the bottom - click on Repair my Computer then click on Startup Repair. ( see this tutorial )
If that does not work, use the command prompts as described - I think you may need the fixmbr and fixboot commands.
Hope it helps