Using VB, how can i prevent the results to be shown if the values inserted are wrong?

For example, if the user inserted in two text fields two values out of range and then he pressed OK, some unknown values such as (Nan) is shown in the result text, what codes shall i use to make warning box that to come out and ask the user to insert values within specific range and prevent the those unknown values to be shown in the Result text?Using VB, how can i prevent the results to be shown if the values inserted are wrong?
I don't know VB, but try this tactic. I know that in Java, there is something called a JFormattedTextField where you can say what data can be typed in, see if there's something like that in VB. Or, you could use a spinner (a text field with two arrows pointing up and down) to scroll through values. If this was in Java, I could have helped you a bit more.