...I have about:blank set, and that's a blank page, dontcha know.

But, if anything it'd be spyware as the culprit.

Download Spybot S&D, scan for spyware, and then get rid of whatever there is.
Then select in tools -> browser pages -> to reset your homepage to whatever you want, and then run immunize, to protect you from the spyware in the future.
Then, FINALLY, in Tools, run IE Tweaks and choose to lock start up page for current user.

That should sort it out.