Get more information about your computer. Go to the Apple in the upper left corner of any window, then "About This Mac", then "More Info..." Copy and paste the information here, but omit the serial number and Hardware UUID (if present).
Leopard 10.5 Technical Requirements - http://support.apple.com/kb/HT3759
Not all older computers can run Snow Leopard.
Snow Leopard 10.6 Technical Requirements - http://support.apple.com/kb/SP575 - note by K Shaffer - http://discussions.apple.com/message/12921514 : "Some early Intel-based Macs can't use Snow Leopard 10.6 installers; of those, the Core Duo (not 2 duo) were suspect and had issues." - and a qualifier by "a brody" http://discussions.apple.com/message/13028822 : "I think that refers to the ones without at least 1 GB of RAM."
Macs and Software that will run with Snow Leopard (Mac OS X 10.6.x) - https://discussions.apple.com/docs/DOC-2455
A Mac OS X 10.6 Snow Leopard Application Compatibility List - http://snowleopard.wikidot.com/
Texas Mac Man, Leopard disc sources - https://discussions.apple.com/thread/3636662
"a brody" post about upgrading to Leopard - https://discussions.apple.com/thread/3373795