"You need to restart you computer" Message during upload

Ok, I've had my imac for a few weeks now with only one problem that's recently occured and the problem is this.
During the process of uploading attachments to emails or posts or uploading anything to putfile, the screen dims from the top and I get a multilingual message telling me to hold down the power button for several seconds. This first occured while uploading a .zip file.
I have absolutely no idea what to do, and would be extremely grateful if anyone could offer any help. Thanks

 

Yes, that's the one. Are there any usual steps to take to solve this?

EDIT

On closer inspection, it appears that this only happens while using Opera, so unless anyone has any ideas, I can always install firefox.