What do you mean pressing 'C' on restart? Are you trying to boot from a system disc or what? Pressing 'C' after the chime only works when a bootable DVD, such as system disc, DiskWarrior etc. is in the drive.
Are you placing the SL disc in the dive, rebooting and holding down 'C' after the chime, is this a retail install DVD of Snow Leopard, an original system disc that came with the machine when new and again, or what? Also if this is an iMac or MacBook, does the optical drive in fact work?