I think that if you choose
repair your computer -> command prompt
then type format C: it will work
If you installed your OS on another drive like D: or E:, be sure to type that specific partition instead of C: so it will completely eliminate the OS and everything attached to it.
If you want to format all of your partitions this will work too, but you'll have to repeat it for each partition.
Now if you want to delete all partitions and create new ones i'm not sure how to do it with a Windows 7 CD.
Hope this helps :]