Is this a dual boot system? Vista and Windows 7 use BOOTMGR, while Windows NT, 2000, and XP use NTLDR. Link to Microsoft sidt for restoring NTLDR, it's for Windows 2000, but Windows XP should be the same.
Note that you can also get a NTLDR error if you have multiple hard drives and the logical driver numbers have changed. This is controlled by your computers BIOS. Usually there's a BIOS setup option that lets you choose the "order" of the hard drives.