Start up repair may fix the problem
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- Remove all floppy disks, CDs, and DVDs from your computer, and then restart your computer.
- Do one of the following:
- If your computer has a single operating system installed, press and hold the F8 key as your computer restarts. You need to press F8 before the Windows logo appears. If the Windows logo appears, you will need to try again.
- If your computer has more than one operating system, use the arrow keys to highlight the operating system you want to repair, and then press and hold F8.
- On the Advanced Boot Options screen, use the arrow keys to highlight Repair your computer , and then press ENTER.
- Select a keyboard layout, and then click Next .
- Select a user name and enter the password, and then click OK .
- On the System Recovery Options menu, click Startup Repair .
If the above steps is not working, Borrow a Vista installation disc from one of your friend and try doing start up repair
http://www.bleepingcomputer.com/tutorials/tutorial148.html
Also you may try starting your computer in safe mode and try doing a system restore
http://windows.microsoft.com/en-us/windows-vista/Start-your-computer-in-safe-mode
http://www.howtogeek.com/howto/windows-vista/using-windows-vista-system-restore/
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