Excel 2007 : Correct input format - How to force users?

Hi All,

I have spreadsheet where users needs to fill in times e.g. 10:00 to 10:30.
My problem is for formulas further on the sheet they need to input time in this fromat (10:00).
Is there anyway I can fromat the cells to give a prompt if the format is not correct?
E.g. if they type 10.00 there should be a prompt or warning advising them of their mistake.

Hope you guys can help out?

Thanx
Woutie