I'm not sure if it is a bug in the .Net Framework or an intended behavior, but when a RadioBtn is focused it also gets selected (or clicked).
When you disable prevBtn the focus moves to the RadioBtn.
You should move the focus to nextBtn and it will be fixed:
Code:
Sub ShowData
If AnsRec=0 Then
<b>NextBtn.Focus</b>
PrevBtn.Enabled=False
NextBtn.Enabled=True
Else If AnsRec=5 Then