I'm using Internet Explorer. I've just checked and Firefox doesn't do it. And yes, it's on EVERY web page. And nope, I don't use Firefox regularly because It's unusable for regular browsing (no zoom), that and there's no Favourites option.
I've just found something out. Every time it happens there's a tiny logo in the corner. It's so tiny I've not been able to make it out until now. But just now I've found out it's actually the iGoogle logo, so it must be something to do with that! Any clues how to get rid of the damned thing?