downgrade to snow leopard

hi There,

I recently upgraded to mountain lion from snow leopard and ever since my computer has been playing up.

Its slow and hangs on boot up not to mention incompatable software which is why i woiuld like to downgrade to snow leopard.

i have a quard core intel xeon mac pro. I bought i fresh copy of snow leopard to install. However it won't let me.

Either i press option key and it just spits it out of the dvd drive or when i have tried to boot up from start up i recieve a kernel panic.

I did back up an older version on super duper but that doesn't get past the blue screen even though i hae repaired the disk permisiions.

I cannot find my original mac pro disks although they would be tiger, however i do have one for my laptop would that work?

any ideas as have run out of them?

thanks