Internet Explorer is definitely mocking me
As you guys suspected, I was indeed plagued by a Trojan and some spywares. So I've scanned and removed them. I thought everything's hunky dory.
Ironically enough, right after my Ad-Aware reported to me that it has downloaded and installed some new definitions, as I was just doing my work... POP!
IE pop-up is set to annoy me for the rest of my life