Hopefully its not seeing C as unallocated, if so then something isn't right, assuming C is where your OS is housed. Did it require a reboot to finish? If so did you reboot.
If nothing is wrong other than it being unallocated then all that means is you just need to create a partition there and format it. Easiest way to do this is through control panel -> admin tools -> computer management -> disk management in 2k/XP/2003/Vista