Go to the RUN command and type MSCONFIG.
You will get a screen come up saying 'System Configuration Utility'.
Click DIAGNOSTIC STARTUP, then click back to NORMAL STARTUP. Then click APPLY then CLOSE. Windows will then ask you to reboot. Click RESTART.
You machine will then restart, when the BIOS has finished checking and it about load windows, a menu should appear with the Microsoft Windows Startup Menu, in a sort of DOS looking mode.
Enter Recovery console, then it may ask you to log onto a Windows directory. You will need the Administartor password for this.
You should eventually get to a c:\windows\ prompt. Type Help or ? i think then, you should see Format. Type FORMAT C:
WARNING! All data on non-removeable drive C will be lost.
Do you want to continue?
Well its up to you?