Here are a few things to try:
Boot from the Vista DVD or a Recovery Disk (see below), then:
1. Run System Restore from a date before you started having problems. click here If System Restore doesn’t work when selected on the menu, go to the Command Prompt option, type rstrui.exe /offline:c:\windows and press Enter.
2. Run the System File Checker. This can repair many corrupted system files. It will also identify in the log file any corrupted files it could not repair click here
3. Run a Vista startup repair. This can repair a lot of problems click here
4. Do a Vista upgrade repair reinstall. This gives you a new OS without changing settings, files, folders, other programs and the like click here Read the note regarding SP1.
If you don’t have a Vista DVD you can download and create a Recovery Disk (32-bit x86 or 64-bit x64) click here
After downloading the ISO image to your hard disk, burn it to a CD (a DVD is not necessary as the 32-bit version is 120MB and the 64-bit version is 148MB). On the CD, there are two folders (Boot and Sources) and one file Bootmgr.
It cannot install Vista but it repairs an installation in the same way as the Vista DVD, i.e. boot from the disk, choose ‘Press any key to boot from CD/DVD’ and Vista will start to load the Windows files. Select your language, time, currency and keyboard then click ‘Repair’ in the bottom left corner. There are five choices within Repair: Startup Repair, System Restore, Backup Restore, Memory Test and Command Prompt.