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.
http://support.microsoft.com/kb/318728
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.