Most common problem with Windows is: It won’t boot anymore.
But don’t worry, Microsoft has that solution, and so do I.

Buy a Vista or 7 DVD, which matches your language, version and Service Pack (downloaded ISO’s will also do the trick, but it’s harder to find the good version).
Boot it.
Now, when it asks your country, language and keyboard, click next.
Now you’ll see “Install now”, don’t do that!
Look for “Repair this computer” instead, and click it.
Select the installation and click Next.
Now click the Recovery Console.
And now, these commands:
diskpart
list disk
Look for the right disk.
select disk #
Replace the # with the disk number (most likely 0).
list partition
Look which partitions you have.
Be sure which partition is Vista/7.
select partition #
activate
I had 3 on a customers laptop, and selected all of them, step by step.
exit
bootrec /fixmbr
bootrec /rebuildbcd
bootrec /fixboot
exit

Now, click Startup Repair.
Click finish to reboot your system, and see it booting.

Source: Ubuntu Forums
Idea of writing: A customer with that problem at work.