ibook G4 being forced to shutdown by virus?

Hi
iMy book G4 will switch on normally and my desktop appears as normal but once it has been on for abour 30 seconds to 1 minute, a grey veil descends down the screen and a message in several languages comes up with the 'button' icon behind it saying that I must force shutdown by holding the power button and restart my computer. I cannot move the cursor or get rid of this screen unless I shut down.When I restart, the same thing happens again. Just before this began, I had downloaded some fonts and was working on my lap from battery power. The ibook was hot and sometimes my fan doesn't always come on so it gets quite hot. what do you think is wrong?

Hi,

A similar thing just happened to me. I was working on my mac, with a safari screen and a firefox screen open, and suddenly, the screen turned white, flickering back to my web page intermittently, with a rolling screen (like on old tvs). Any ideas what this might be? My employer is experiencing a district wide virus, but I was told Macs would not be affected. . .

Thanks for any feedback.
Nina