Standard combo box doesn't show text when has focus

I'm building a simple windows app. with a couple of combo boxes which are bound to a SQL Server table datasource.
WHen I run the app. and a combo box gets focus, the text part of the combo box is highlighted in blue, but the text is not visible.
I can see all the options in the drop-down and can select one, but the text is not visible until I move focus to another control.
I was using .Net 3.5 and VS 2007, but have upgraded to .Net 4 and VS2010, but still have the problem.
Fore Colour is black and back colour is white.

Any help would be great.
Thanks, James