You can try booting the install DVD and doing a Repair My Computer, then select the Command Prompt and run
chkdsk /r. See if that fixes any file system errors and allows you to boot again.
How to run System File Checker (SFC) or chkdsk from the Recovery Console
- Boot your Vista or Windows 7 installation DVD
- When you see "Press any key to boot from CD or DVD", press Enter
- At the "Install Windows" screen, click on Repair your computer at lower left
- At the System Recovery Options screen, make note of the drive letter assigned to your boot drive (normally C:) and click Next
- At the Chose a Recovery Tool window, click on Command Prompt. You will be sitting at X:\Sources directory
- Run SFC or chkdsk
- If you did not note the drive letter of your boot disk, you can enter bcdedit and look at the osdevice line to see what it is.
- For sfc, type sfc /scannow /offbootdir=c:\ /offwindir=c:\windows and press Enter (use the letter from above)
- For chkdsk, type chkdsk c: /r and press Enter (use the letter from above).
Let either run to completion undisturbed.
Vista or Windows 7: If you don't have a install DVD, you can download a legal copy here:
Official Windows 7 SP1 ISO from Digital RiverDownload Official Windows Vista RTM with SP1 Setup Files (32-bit and 64-bit)Make sure you get the same version you have installed: 32 or 64 bit; Home Premium, Pro or Ultimate.