The alarming part about it is that Microsoft Security Essentials found nothing, neither did a few other antivirus programs I tried using. I got rid of it by restoring my computer to an earlier point. Go figure.
I’ve seen it before — in my case it was nothing more than a Chrome addon/plugin that would scan any web pages you visit for keywords and then add advertisement overlays to it — like what you showed, @chrisburton. Accidentally installed or with malicious intent, of course MS Security Essentials wouldn’t detect that cause being an addon, it’s “legit”.