|because Windows (any version) should normally go to C drive|
Doesn't make the slightest bit of difference. Windows from XP onwards, both syetem and boot drives, can quite happily reside on any drive from C: to Z: The only necessity is that the system drive has to be a primary partition. It was only Windows 98 and earlier that had problems on drives other than C: and even they can be circumvented.
The easiest way yo get rid of Windows XP is to boot from Windows 7 and delete it from there as you would delete any other files. The startup files for Windows 7 will probably reside on the C: drive so formating drive C: will stop Windows 7 from booting.