Major problem on used mac. Please Help

Hello,
I'm new to forum (and new to macs as well), but I was hoping to get some help with my wife's powerbook g4 we just purchased on ebay. It is a 12 inch powerbook g4, with 1.0 ghtz processor. The problem started last night. She was using the computer normally (not taxing, just looking at some websites) and then the computer screen froze up and scrolling lines of code started to vertically move up on the screen. The code enveloped the whole screen and there was nothing we can do to start the code from scrolling. We had to shut down the mac (by holding down the power button) and start it back up. Has anybody experienced this problem before?