Worrying Blue Screen When Shutting Down


I'm running a year and a half old powerbook g4 with just under a gig of ram. Recent additions include a Lacie firewire drive and a USB modem.

I've recently started getting programs stalling (particularly firefox) more worryingly, when I shut down, or switch from ac to battery power, the computer freezes, with a blue screen and a watermarked large power icon and a warning in several languages

"You need to restart your computer hold down power or press restart button"

The computer freezes, and often won't restart for several minutes.

I've very much like a speedy response to this.

Warm regards.