There's hundreds of reasons why you would need to repair your master boot record: Installed Linux, dual booting with any OS, corruption are a few of the biggest reasons you'd have made changes to your MBR. How do you restore it to your original Windows XP one you ask?

In these instructions I'm using the operating system is on hard disk 0 AKA drive C.

Furthermore this guides assumes that your operating system is on Harddisk 0, drive C:\)

First of all Windows XP Recovery Console Installation Guide You'll have to have it installed ;)

When you reboot make sure that you select the recovery Console. If it's not installed and you can't get into windows to install it then you'll have to boot to your windows XP CD to get into the Recovery Console.

1. When the Recovery Console prompts you or which system you want to start type 1 and hit enter.
2. After the Console has finished loading type in the following:
   fixmbr /device/harddisk0
3. You'll see some warnings and just type y which answers yes to all of the prompts
4. After that you can restart and you're back to using the default MS boot loader, booting to the right hard disk now.