Progress Bar / Updating Form From Another One (Graphical Glitches)

My form has FOUR comboboxes populated from text files. The text files are loaded into an array into their own background process once completed are added to the combox. Due to the sheer amount of items the loading causes a lag and as the combobox are on the main GUI thread a glitch appear for several seconds. I then had an idea of loading the initial form, showing another form which is made the topmost form with a progress bar (values derived from the count of items in the arrays populated from the text files).

When I added the progress bar any other label or image on the SAME form did not show until the progress bar had completed, how can I resolve this? When I use it in conjunction with the other form i.e. using the form with the progress bar to populate the other in the background graphical glitches appear (mad transparency effect). I have the code from the form with the progress bar below:-

'Load City information
'*********************
'Set form title
Me.Text = " Application loading city lookup information ...."
'Declare string array to store list of towns
Dim citylookup() As String
[Code] .....

Why I experience the glitch (assume it's because of refreshing the item, some kind of repaint needed)? Any alternative solution to prevent the user from using the form until all the comboboxes are populated?