Javascript - Validating Against Checkbox And Validation

I'm using asp.net validation controls which work fine. I've since introduced an HTML checkbox which should be ticked before the form can be submitted. My function for this reads as follows:

function terms(form) {
var terms = document.getElementById("chkTerms").checked;
var errorsArray = new Array();
if (terms == false) {
//alert("Terms not checked (works)");
errorsArray.push("You must agree to the terms and conditions.");
if (errorsArray.length) {
document.getElementById("termsOutput").innerHTML = errorsArray.join("
") + "<br />";
return false;
}
}
else {
document.getElementById("termsOutput").innerHTML = "";
}}

It is called as follows:

<asp:Button ID="btnSubmit" runat="server" Text="Submit" OnClientClick="return terms(form)" />

The problem I have is seems I can either use the asp.net validation OR the custom checkbox validation I wrote. After the form posts however, the asp.net validation fires!