I have a separate computer set up with Windows XP and IE6 and 7. The dropdown menu items in IE7 disappear when the mouse hits the 'border' between the items.
I have received three emails from people that they are experiencing the same thing, and in all cases they have Windows XP and IE7 (I asked, and 2 of them are at work on company networks)
I also signed up for a trial at crossbrowsertesting.com, as you can connect to a live system for testing with them... In the Windows XP SP2 and IE7 environment, the same issue occurs... horizontal menu dropdowns disappear when you try to mouse over them.
Let me know if there is any further information I can provide to help find a solution...