When installed on a single drive, with only an optical drive attached...Windows installs to C:.
Any drives/partitions created after that start with E: and move through the alphabet in order, unless user decides to use a letter farther down (such as Y).
When doing a multiboot...it really doesn't matter what drive letter is assigned a partition created for ensuing Windows installs. Example: I dual-boot XP and Win 7. When I am in my XP install, my Win 7 install is viewed as E:. But when I am in my Win 7 install, it is my XP install which is viewed as E:. This occurs and is normal...because whatever version of Windows which is booted into...automatically is viewed as C: from within that install.
At that time, all other possible installs of Windows...are viewed as nothing but storage partitions/drives and the letters assigned them from that view really don't matter when booting into a Windows install.
If the system has hidden partitions or has partitions created by an OEM...so that the Windows partition appears to be something other than C:...that just adds to the confusion. But the key is to remember that any Windows partition which is booted into...will always be C: when looking at all other partitions that exist from within.
Note: Each time that Windows is installed, it takes inventory of existing partitions. If it sees that you have 3 partition and an optical drive, it will assign the letter G to the partition created. But...when that partition is booted into, it will refer to itself as C: and change the drive letters it assigns to all other partiions. It's an illusion of sorts but it's the way I see Windows working.
Edited by hamluis, 31 January 2012 - 10:07 AM.