The default size for the menu that drops down from a combo box is 0 in Win2k. So if you click the arrow to drop down a menu (such as the "Source" and "Destination" paths in the Rebuilder screen) you get a useless little sliver instead of a menu.

Microsoft apparently fixed the problem in WinXP and set the default value to something reasonable. I haven't tried ClrMAME under WinXP, but I've seen this bug in other apps and I'd be willing to bet it works fine there.