I keep getting 'you need to reboot your mac'

I keep getting 'you need to reboot your mac' with a grey-out background. The message is in several languages. Is this a virus of some sort? It's been years since I have worked on a Mac and I picked up this Mac OS X Tiger 10.4 system. Lately I have been getting this notice (takes up the whole screen) stating I need to re-boot.

I did recently try installing and uninstalling various versions of the flash plugin, but that is the only change I have made to the system.

Any clues?

Thanks.