Did you re-install Windows on the C: Drive and not just format it? Which version of Windows please? (What follows should work with Windows 9x and ME, but probably not with 2000 and XP.)
If you have installed Windows 95, 98 or ME what might have happened is that Windows has reset the partitioning. If that is the case you can use a programme called MBRWorks to resurect the lost partition. It's a freebee available somewhere on the web, but it's small and I can email you a copy if you want.
Whatever you do back everything up to some removable media so you don't lose any more data!