Dialog Box Busy

Hi,

My dialog application uploads a file to a microprocessor system through an RS-232 port.
Clicking a button starts uploading.
While running the application doesn't accept any button clicks.
A progress bar is following the upload procedure.

The problem is that very often the progress bar freezes and a sand clock shows up. The process always goes to the end and eventually the progress bar gets completed but but meanwhile everything stays frozen.
The same happens if the user tries to move the dialog while running or change the focus out of the dialog.

Does anybody know how to fix similar issues?

Thanks,
Ivan