Migrate easy 7 - new system disc has 'wrong' drive letter

I am running XP and I used migrate easy 7 to clone my system disk from drive D to drive E. Initially I left both drives in the machine and found I could boot off either depending on where I set my BIOS to boot from. The problem is the software on the cloned drive expects to be on drive D.

I removed drive D and swapped E: to the same connections drive D: was using, but it still boots up as E:.

How do I get it to come up as drive D: (There is a microsoft paper that tells me how to do it by editing the registry, but is this what I am supposed to do or have I done something wrong? The manual did not mention anything about this)

I intend going back to the original drive as system disk and repeating the process once I have a clear way forward, so any help would be appreciated.

Thanks
Chris