Closing a modal form switches to wrong application

Question

The situation is as follows:
I have an MDI application, several windows are open.
Apart from my application, I have other applications open such as Outlook and Internet Explorer
My application opens a modal selection form. (form.showDialog()).
When the selection is made I hide the modal form. This gives focus back to the main application form.

however

Part of the time (not all the time), after hiding the modal form, the application does not go back to my main form, but switches to a completely different progra, like internet explorer, or whatever is open. This problem did not occur on my development machine. My clients have slower systems then my development machine have reported this to me. After investigation, I can clearly see that slower computers have the problem much more often, up to 90% of the time.

This is all under windows XP. Under windows vista the problem occurs even on high-end machines.

Any help on this would be MUCH appreciated, as it is a big problem for my client. Obviously.

Thank you,

Jagg