What happened in this case is that Windows XP installed
NTLDR
NTDETECT.COM
BOOT.INI
on the primary drive and the OS and other files were installed on the second.
You will have to do several things:
1. copy those three files to the drive that has the OS on it (note: They are hidden system files)
2. use a
partition editor to mark that drive as bootable (active)
3. edit the boot.ini to reflect the drive change (which actually may not be necessary if you merely put the secondary hard drive in the primary's place (IDE or SATA port))
4. reboot after removing the primary drive.
OR you can remove the main drive, then boot into windows setup from your Windows XP CD and
1. Press the R key when prompted to start the Recovery Console.
2. When Recovery Console starts, it will prompt you to enter a number corresponding to the Windows XP installation that you need to repair. In most cases, you'll enter "1" (which will be the only choice). If you press ENTER without typing a number, Recovery Console will quit and restart your computer.
3. Enter your Administrator password. If you don't enter the correct password, you cannot continue.
4. At the Recovery Console command prompt, enter "fixmbr" (no quotes) and then verify that you want to proceed.
5. Repeat #4 but use the fixboot command
You should be fine after that. Worse case scenario is that you'll have to do a Windows XP repair installation, but the other steps should work in any regard.