experienced mac users I need help

[bignose]

I bought a imac 6,1 on kijiji that I was going to use for internet and movies for the kids. But I'm having troubles with graphic glitching and locking. The last owner upgraded to snow leopard osx and left discs from his 15" laptop (osx 10.5.5) in the box. Soooo I didn't get the original os and from what I've researched so far snow leopard is not compatible with my iMac. I think it's a software problems but I'm new to Apple.
I've owned a iMac 9,1 for a couple of years and have never had a problem but I'm stumped with this computer. I feel ripped off at the moment so if you can help I will give you a frag from my tank.