[DotNetDevelopment] Control validation based on another control

[Stacie]

I have a textbox (OtherHours) and listbox (OtherHoursType) in my web
form (using VS 2005/VB.net).  If the user enters hours into the
OtherHours control then they must also select an Other Hours Type in
the OtherHoursType control. I'm having difficulty figuring out how to
write validation for this.  Does anyone know how to do this or have a
good online reference to recommend?