Access 2007

I have an Access 2000 database that I have opened in Access 2007. One issue
I have encountered is the triangle on my combo boxes is invisible. When I
click on where the triangle should be, it reappears and I can select the
items from my list. However as soon as I click out of the field, the
triangle to the combo box disappears again.

Any thoughts on how to keep the triangle visible?

Thanks for your help.