ummm hopefully im understanding it correct....
sounds like D: isn't currently setup as a system drive. even though windows might be installed on D: doesn't mean it was the boot drive.
if the drive is still accessable. with the 100 gigs on it. unplug your C: drive and reinstall windows on D: hopefully the partition is intact and you can actually do it. Hopefull the setup should make that drive the system drive.
originally the system files were all on C: drive despite a dual boot system. the important stuff was on C: telling it you had an OS @ c:\windows and at d:\windows with all sorts of other special files. got rid of that and well the computer didn't know what to do.
any way try reinstalling the operating system. BUT you dont need to reformat if your partition is intact. So skip any thing about formating.
hope that helps.......................

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