Formattin Meh HD

Im thinking of formatting my computer, I have so much crap on this machine, and now my sound just stopped working.

I have 2 HD’s (both built-in) as the C drive and D drive (120gb each). If I format, will it just wipe the C drive and leave the D drive intact, or will it wipe them both? Or can I choose wether to wipe the D aswel as the C?