Drive letters cannot be re-assigned

I had, amongst others, two drives labelled D: and E: on my PC and wished to exchange the drive letters. I got as far as temorarily renaming drive D: to X: but then had to divert my attention to other matters. Since drive D: was not actually in use this had no immediate consequence and I thought no more of it until recently when three MS Updates for Office 2003 repeatedly failed to install. I still did not connect the two until, whilst researching a solution to the failed installation problem, I came across MS Article ID.903776 which states quite clearly that the renaming could well be the cause.
I now needed to rename X: back to D: but, on entering Drive Management, I found that D was no longer offered from the drop-down list. Is there a way around this, please?

Many thanks to whs and kodi for your replies. However, I now have a solution.
I have used PCWizard on various machines for years and, whilst checking other aspects of my hardware with this for other purposes, I noticed that this programme showed there to still be a D: drive whereas the Win7 Disk Management etc systems did not. A quick check showed that the "new" D: had been assigned to my HP Photosmart D7260 printer. Simply unplugging the printer allowed D: to "disappear" and I could then rename X: to D: via Disk Management. The printer is now recognised as H: drive.
Now....what's the next problem....?