First boot on your Windows Vista/7/8 installation DVD.
If you have one of the many OEM computers that didn't come with a Windows installation disk, you can get the same effect with a Windows repair disk, which you can download (eg Win7-32bit, Win7 64bit) or create from another Windows Vista/7/8 computer (see this page for Win7 and Make your own Windows repairCD, and Windows 8 UEFI repair USB must be FAT32, other Win8 guide). When you get to the Regional settings, select your Location/Keyboard setting then click next. On the next page, click on "Repair your computer." On the next page, if it finds your Windows installation, make sure it is UNSELECTED before clicking next. Then click on "Command prompt". From there, type in the following 2 commands:
bootrec.exe / fixboot
bootrec.exe / fixmbr
Now close the two windows and click "Restart." Take out your Windows DVD and hopefully, you will be left with your Windows bootloader.