From how you described it, it looks as if your mate has the drive with the MBR (boot record, the bit the PC uses to find and boot a Windows installation). If it just has the Windows files and folders it won't work. If C: contained the boot.ini file, then it would probably try and boot from that then boot your Windows installation on your D: drive. You could try going into the BIOS by tapping DEL (or whatever key it tells you to press to enter setup) and changing the boot order so that it boots from HDD-1 instead of HDD-0. Or you could plug the D: (which will now be C: as the other one has been removed) in exactly where your old C: was and see if it boots up. Usually this is in the primary IDE socket on the motherboard (the opposite one to where your CD/DVD drive is plugged into) and set the little jumper on the back to master.