Issue with list box(Drop down)

Hi Experts,

Issue with list box(Drop down) which I would like to mention clearly here below.

I have a drop down field in my screen(Module pool) which includes the text(values) as @Non billable@ & @Billable@...When I select Billable(from the drop down) one of the field named @Progress notes@ in the same screen(Module pool) should become "Mandatory"  and When I select Non-Billable(from drop down) one more field named @R&D@ in the same screen(Module pool) should become "Mandatory".....

How can I rectify this..Please help me out.

Regards,

Naveen.B