Form Not Visible

I have a very simple form with two combo boxes & two text boxes that feed
selection criteria to a query & then a command button that opens another form
based on the query. This form is in the front end of a database that I have
installed on multiple users's PCs, some using 2007 & some using 2003. Some
users can open the form just fine while others seem to be unable to open the
form. However, when I tried to delete the form, I got a message that Access
was unable to delete the form while it was open. So it's opening--its just
not visible for some users even though I have installed all of the user's
front ends from the same source & uninstalled/re-installed repeatedly. Two
of the users unable to view the form are using 2007 & one is using 2003. Any
ideas?