Made a drive bootable by mistake, how to undo?

I was working on a machine, and needed to fix the boot record on the internal drive; it kept trying to load Windows 7 Setup rollback despite the rollback not doing anything... unsure of what happened there in the first place, but that doesn't matter.

I looked up how to use bootsect off of the W7 DVD, and in my haste I copypasta'd the command on the tutorial, which was essentially to replace the boot sector on all drives... whoops.

Now my external drive is bootable, and it seems because of that, and how the BIOS was set up, in Windows XP (fresh install) the external drive (not the system drive) is set to C:\, and the system drive is set to D:\. What do I do to rectify this, namely what the hell do I do to make my external drive unbootable again? All of the stuff I've looked up so far says to use a program to replace the MBR... I don't want it replaced, I want it GTFO'd.

Reformatting said external drive isn't an option, I have critical data on it and nowhere else to put it.

Im not sure if its the same in win7 but last time i had this happen i was able to change the boot settings from MSCONFIG, open up task manager click file and then run new operation/process and type in MSCONFIG.exe goto the boot tab and there should be a part there for editing your boot.

That would be boot.ini, not what I'm looking for. I gave my external hard disk an MBR, a Master Boot Record, which is invisible to the file system.

Try editing it in the BIOS maybe.

hirens boot cd. it has the tools you're looking for.

^That, or a GPartEd livecd, which is what I have experience with. You boot off of the CD and it gives you a nice GUI for editing the partitions on your drives, including marking them as bootable or not.
E: Do be careful, whatever you do, because in my understanding any tools capable of doing this will be primarily focused on creating and/or formatting partitions, and as you said you have critical data.

I have (and have used) Hiren's BootCD, I just have no idea how to work the MBR tools and don't feel comfortable with using tools I can't work on a drive with important data on it.