Still with the negative margin? I thought I'd looked at this issue once but I can't seem to find the original thread.
Oh well, ...
IE7 has issues with negative margins. Your -20px left margin, in other browswers, is trying to compensate for the default left padding on the ul.
The solution here (I hope), instead of padding it the li's right and bringing them back with the negative margin, would be to eliminate the default padding.
Try this -
margin: 0 15px;