Trying to identify a locked up iMac
EveryMac has a fabulous utility on their site to identify any Mac from its Serial Number or other identifying marks.

In this case, it looks like you have a Late 2006 iMac Core2Duo 2.6GHz 24"



If you don't have the original install disks, then your best bet would be to buy Snow Leopard directly from Apple. The latest OS the machine can run is Lion 10.7.5, but that is a download only. You'd need 10.6 first.