How do I format the hard drive C, in this case?

I fixed the LAptop, and Now I want to format the hard drive,. . I have windows XP sp2 media center edition 2002,The hard drive has two partitions,. volume C and volume D. the volume C,. has a capacity of 30 gig. but it says theres only 23 gig. available,. which is weird since I recovered the system to a factory defaults. I am looking inside the C voume,. and I dont see anything that has 7 gigabytes of size.-When I frst got the pc, I had 29.5 gig of free space. Anyways,. I right clicked the My Computer icon,. ...I clicked "Manage",. and I followed the steps in order to format ,. but it wont let me do that,.. the "format" opcion is shaded,,,instead,. the other volume,. ( D) is not locked and I could format it ,if I wanted to do so.

I also went into the command prompt,.. and I typed "format c:",.. I put the volume's name,. and finally says that there's another process that wont let me format it., and I dont know what to else to do.
Thanks for your help.