Mac OS X Forced Shutdown

I' thought it was a 1 time thing but it happened again literally a few minutes ago..I was working in Photoshop on my iMac (with the newest leopard installed all updated) editing a photo and when I went to save the .psd this grey bar scrolled down the screen (i could still see my desktop and what i was working on) then a grey box popped up in the middle saying "You need to restart your computer. Hold down the Power button for several seconds or press the Restart button" then some next languages underneath that. I have absolute no idea what is wrong but it is SERIOUSLY annoying when I've work for hours on a photo then when I'm ready to save this crap happen.

I actually took a picture on my iPhone of the screen but it's taking FOREVER to upload to my computer. I'd gladly appreciate it if anyone can tell me what's up and how the hell to stop this..

thanks!