Optical drive not working!

I have a white 13" macbook (from 2007 I think). Im currently using os x leopard and have bought a cd to update to os x snow leopard. But when I put in the disc, the drive just whirrs a lot then spits the disc back out! I've tried resetting the SMC, but nothing changes.

Theres another weird thing the computer does which I suspect is related - the fan often runs very fast, when all I'm doing is browsing the internet.

Can anyone help? Do I have to replace the optical drive?