Just to be safe, try downloading MalwareBytes' Anti-Malware
and giving it a scan with Internet Explorer closed. While Internet Explorer isn't the minefield of bugs and swiss-cheesed security holes it used to be (it's actually rather secure now by most measures), it's still fairly vulnerable given the focus on it in the field of malware. It's not unthinkable that something managed to attach itself to your browser over the course of your web browsing sessions. In addition, most attacks against IE occur at random after you first become infected, so as to mask which website was actually the one that had the malicious code injected into it.
It might have nothing to do with malware, however. You might simply need to repair your IE installation (you can do this by "uninstalling" it from Add/Remove Programs (or Programs and Features if you're running Vista/7) and downloading it again from Microsoft), or even just clear your history or temporary internet files.
EDIT: I'm also reading in a knowledge base article
that this problem is fairly specific to IE7 - Have you not upgraded to IE8 yet? If not, you should give it a shot. It's a much better browser overall - Actually standards-compliant, unlike 6 and 7, and much more secure (7 is still extremely vulnerable to malware attack). Of course, it's easy to just say "use Firefox/Chrome/Safari/Opera/(insert browser here)", and I do recommend that, but if you have your reasons and you're using IE8, there's no pressing need.