Don't know if this will help, but I had that problem a couple months ago when I installed EHM on Windows 7. I could scroll down, but not up. Only in EHM.
What I did to fix it was I had another mouse I used (different brand/model) I plugged that sucker in and Windows installed some new drivers for it. I booted up EHM and the scroll up was working again. Unplugged the other mouse, and plugged mine back in and worked like a charm.
Big question is, are you having any other problems scrolling up in any other programs, and if you uninstall your IE extension does it go away?