If you need to restore the Windows boot loader after using GRUB (or something similar) and subsequently deleting its installation, you can use the Windows setup disk. The following refers to the Windows 7 installation media, but Windows 8 should have similar options from the disc.
Boot onto the disk and click the "Repair your computer" on the install screen that eventually comes up. If the only problem is the bootloader, it should automatically detect this and restore it.
If you are running an earlier version of Windows, or it doesn't (for whatever reason) automatically restore the bootloader, you'll have to do it manually.
Boot to the CD again and go through the prompts to get you to the "Recovery console". From here, enter the following command:
This will reinstall the Windows bootloader for you, automatically detecting the location of your Windows installation.
Once it completes, restart your computer, and you should see Windows again.
However, if these steps don't resolve the issue, you might have to dig deeper. Note: additional info is taken from Microsoft KB927392.
You can rebuild the bootloader completely if necessary, by exporting and then deleting the existing bcd, and then doing the rebuild step, again. In the recovery console, type:
bcdedit /export C:\BCD_Backup
attrib bcd -s -h -r
ren c:\boot\bcd bcd.old
If the OS doesn't get detected, you can try using
bootrec /scanos to have it list all operating systems on all disks compatible with the bootloader, including those not in the BCD store.
bootrec /fixmbr will rewrite the Master Boot Record to the system partition. This might be useful if it becomes corrupted.
bootrec /fixboot will write a new boot sector. This might be important if it was replaced with a nonstandard boot sector, was damaged, or an older version of Windows (before Vista and 7) was installed alongside the newer version.
When doing these operations, it's important to note that you're changing core elements of your system disk. Always make sure your data is backed up to a separate disk before doing anything that might potentially cause data loss. These steps won't necessarily cause you to lose your data, but prudence is always important. As they say: "Hope for the best and prepare for the worst."