Changing drive letter of HFS drive in windows destroys partition table

There's no way to stop a user with Administrator rights from doing this - if you mess with the hard drive at a low level, you always risk breaking something. This is doubly true when you're using tools that don't comprehend all of the disk structure that's in place - in this case Windows tools with HFS+ filesystems.

I'm surprised your HFS+ partition(s) had drive letters at all - mine say "unknown format". Are you running some software in Windows that allows visibility of Mac formatted volumes? This may fool Windows into thinking it knows what it's doing with them - and by the sound of it, it's writing FAT blocks to those partitions when you change the letters.

I take that second part back - when in BootCamp rather than VMware's virtualisation of it, I get HFS+ partition visibility too.

I still stand by the first paragraph.

When you are running Windows and you change the drive letter of a HFS volume, the data on the volume is NOT destroyed. What happens is that Windows changes the GUID code of the volume, causing it to no longer be bootable as an OSX startup volume. If this happens it's entirely fixable, though the fix is a bit complicated. Read this Discussion for the solution:
http://discussions.apple.com/thread.jspa?messageID=11227763&#11227763

FYI, Windows doesn't natively read HFS volumes. The ability to read HFS volumes is conferred when you install the Bootcamp drivers for Windows (I've seen this for both XP and 7). And it's a new feature of the Bootcamp drivers that released with Snow Leopard...earlier versions of the Bootcamp drivers did not allow Windows to read HFS volumes.

I had this exact same problem when I got Windows 7 (Ultimate x64) in October of last year. I'd had Snow Leopard (which came with the new Bootcamp drivers) for about a month, but I didn't know about it until I erased my Bootcamp partition to install Windows 7. After installing the Bootcamp drivers in Windows 7, I noticed I had access to my Mac disks - so I rearranged their drive letters to my liking, and sure enough, OS X was no longer bootable. I believe I was the first to post about this here, since I didn't find any other threads, and it was only a month after OS 10.6 came out - my post is surely long archived now, but you might still be able to find it. Anyway, I came to the same conclusion, but since I didn't know of any fix (Disk Utility found the problem but failed to repair it), I just reformatted my Mac boot disk, reinstalled OS X, and restored from Time Machine, which worked beautifully.

My long-term solution was to disable the Apple HFS driver for Windows. Considering I was able to screw up my Mac disk in the first 5 minutes that Windows had access to it, I deemed having this feature to be an unacceptable risk...