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#11
Do you have the installation disk for your system? If so, boot from it, click on Install/Select Language at the first screen - then, on the second screen, click on Repair this computer in the lower left of the window.
Then you can access Command Prompt from there - and see if it'll let you run it (the volume is not supposed to be write protected).
If it offers you an option to dismount the volume, or to schedule it at reboot, let it do it.