Constant Freezing on iMac A1174

I recently purchased an iMac A1174 (2006 model) from someone, and it was not restored to factory settings nor did I receive discs with it. I am able to use it for about 5 minutes, but the entire operating system freezes after this amount of time (no matter how many apps/tabs I have open) and I have to power it off with the power button to reset it. This happens without fail every time I try to use it.

It is running Snow Leopard. Does anyone have any suggestions on how I can remedy this situation? I hope I didn't waste my money on this, I really wanted to be able to use it.