Full screen ads

I noticed today that my blog seems to be hosting full screen ads. At first I thought it was my PC that was infected, but the ads were only showing on my blog, and I know that I didn't set them up. So either the whole of Blogger, my site, or one of the widgets I use has been hacked. I apologise for any inconvenience caused by these ads and I am trying to remedy this problem as quickly as possible.

[Edit] I have removed a number of widgets from the blog, none of which were essential, and as far as I can tell the ads have stopped appearing. If you see any pop-up ads on this page, either full screen transparent ones, or those that open in another window, please leave a comment on this post giving details about the ads and I will continue my investigation.

Thank you