When you install Linux or another OS alongside Windows in a dual-boot configuration, the installation often sets up a boot loader like grub to manage the booting alternatives. This can sometimes break the Windows boot loader, which can cause problems in certain situations.
One situation is where you have a Windows hard drive and a Linux hard drive. It works fine as long as both drives are installed, but say you remove the Linux hard drive: you would expect the Windows drive to boot up straight into Windows. But instead you might get a message like “BOOTMGR is missing”.
Another situation that I ran into was that I could not upgrade Windows 7 to SP1 in my dual-boot configuration. Reasons why are discussed here. I had to revert the boot loader back to booting directly into Windows before the Windows 7 SP1 update would stick.
There are also a few other situations that might cause your Windows boot loader to get corrupted, and you probably don’t want to resort to restoring your entire drive from a backup image or re-installing Windows from the ground up. So here’s one relatively easy way to fix your boot.
Repairing the Windows boot loader
Here is one way to repair the Windows boot: Find a Windows Full or Upgrade install disc. Boot from it. When it comes up, select your language, then at the next screen, select the Repair Windows option at the bottom of the screen. Select the Startup Repair. Indicate which version of Windows you would like to repair, if it finds more than one installation on your hard drive. It will do some checking, and should find that your BootMgr is in need of repair. Tell it to repair the problem. After that it should boot directly into Windows again.
This works with a Windows 7 Upgrade disc. I’m not sure about Windows XP.