Green Screen MacBook Pro - locked

Hi,

For the second time in 15 days, my MacBook Pro got a complete green screen! The computer stayed locked and I had to reboot it pushing the power button.

In the first time I was working with excel for Mac and browsing in Firefox and logged in Skype. In the second time I was using Skype (in a call), Firefox and accessing a remote desktop through TeamViewer.

In both situations, everything stoped sundendly and the screen got completely green. I tried to use the mouse and other buttons, but anything worked.

Anyone knows what this can be? It is a problem related to OS or it is a hardware problem?

My MacBook Pro:
OS - Leopard 10.5
MacBook Pro 13", with 4GB RAM, and 250GB HD

Thanks,

Márcio Fleury