Deep Freeze is a program used by businesses to prevent changes on computers from being permanent. Apple Retail stores use this program, actually — it's what allows customers to play with the demonstration computers on the sales floor. Then, after store hours, the computers restart themselves and clear off any changes customers made during the day. It's a very popular program among computer stores for that reason.
The fact that Deep Freeze is on there on there makes me wonder if the computer was stolen.
But hey, maybe not.
Either way, how to get it off is your question. So the thing is that Deep Freeze is very difficult to remove. (Obviously, what good would it be if customers could just take it off?) So there's only a few ways to do this. Probably the easiest is what yakyu mentioned:
If you have the installation DVD, you should be able to re-install Snow Leopard.
So for starters, do you have the discs that came with the computer? Or, do you have a copy of Snow Leopard?
If not, there is one other method of removing it, but it's quite difficult...
