A little over a year ago I created two ASP.NET validation controls that are missing from the ASP.NET Toolbox: CheckBoxValidator and CheckBoxListValidator. As their names imply, these validation controls validate a single CheckBox (ensuring its either checked or unchecked, depending on a property setting) and ensures that a specified minimum number of options are checked from a CheckBoxList. For some reason, the built-in ASP.NET validation controls do not validate against CheckBoxes.
I recently updated the CheckBoxValidator control so that a page developer can optionally associate a Button, LinkButton, or ImageButton with the validator. In such a case, the CheckBoxValidator injects client-side script so that the associated button is disabled when the CheckBox is in the inappropriate check state, and enabled otherwise. The following screen shot illustrates this concept. On the left, the CheckBox is unchecked so the Submit button is grayed out; on the right, the CheckBox has been checked so the Button has become enabled.
You can read more about this enhancement at Disabling the Submit Button Until a CheckBox is Checked. The complete source code and a simple working demo is available here.