I have a userform in a workbook where i want the user setup a time interval with 2 comboboxes, one for starttime and one for stoptime. The comboboxes have a list with a range of time in the format h:mm:ss, eg. 13:48:30. The problem is that when the user select a time, it will automatically be converted to decimal format(eg.0.57)in the userform (The user see the selected time suddenly being converted to decimal format). How can i prevent that and keep the format h:mm:ss

Try something like the code below in afterupdate of the textbox:
TextBox1.Value = Format(TextBox1.Value, "HH:MM:SS")

You'll probably want to initialize it in the form.activate:
TextBox1.Value = Format(Now, "HH:MM:SS")

NOTE: This doesn't do validation so invalid values, such as non-numeric characters, can still be entered into the textbox. I'd suggest checking the value and displaying a message if the data entered into the field is incorrect.