Sounds like what you did is boot to Windows with 2 hard drives installed, both of which has OS installed on them. In that scenario, Windows automatically creates a dual-boot system. When you removed the other drive, the remaining OS still had the dual-boot file on it.
If you go to system properties, advanced, startup and recovery, you will see a drop down listing both OS's.
I think there are other ways to do this, but the way I know of is to edit your boot.ini file. You have to enable viewing hidden files in folder options.
The file will open in a text editor, and you will see something like this:
multi(0)disk(0)rdisk(0)partition(1)\WINNT="Microsoft Windows XP Home" /fastdetect
Except you will see different entries because you have 2 OS listed. You need to remove the non-operating OS entry and the edit the relevant disc/partition references. It's not hard to figure out what to do. The bogus OS entry will probably be shown as being on "disc1". "Disc0" is always the 1st, or main disc.
Needless to say, if you screw up, you won't be able to boot, so be prepared. Save a copy of the original boot.ini on a disc so you can copy it back if you need to.
Doing a repair install will also fix this, but is a bit of an overkill.