Thanks for the information provided.
One of my customer who has the same problem told us that the problem is related to IE 7. The problem does not exist in IE 6. The problem disappears after he uninstalled IE7 on his computer.
This problem was due to the upgrade to Microsoft Internet Explorer 7, but NOT the bug of Sothink DHTMLMenu or Tree Menu program, especially NOT caused by DHTMLMenu V7.0 or Tree Menu 2.0 (both latest versions).
Please check details at:
Hope the information will be helpful to you.