Keeping with presence's answer, you will need to reinstall Windows Vista. You could try to do so from your recovery partition, if you have one. By pressing F8 during boot, as if you were going to boot into Safe Mode, select instead "Repair my computer". From there you should be able to run the recovery software. (Depending on the manufacturer, the recovery might be used in a different way.)
Or else, you can reinstall Windows Vista from a bootable USB. Look at the sticker with the product key that should be attached to your computer. It should indicate which version of Windows Vista you are running (Home Premium, Business, Ultimate, 32-bit or 64-bit).
You can download the official Windows Vista setup files from here (instructions included, you will need your product key during installation):
Only Windows Vista Ultimate is available, and only in English, French and German (I would always recommend choosing English anyways, even if you speak German). Link with instructions on how to create the bootable USB is in the same webpage. Once the Windows Vista install USB is created, if you need to install a non-Ultimate version, delete the two lines near the bottom of the \Sources\Inf\SETUP.CFG file in the USB. They look like this:
More info about this: http://jimcofer.com/personal/?p=6119
If you have any doubts just ask.