How to bypass onsubmit event when post back DDL event?

Hello, friends, 

In the source code, there was a JavaScript function isValidInput() for form onsubmit event, i.e., onsubmit="return isValidInput()"

In VS2003, this function was not called when SelectedIndexChanged event in DropDownList boxes was fired.

In VS2008, this function was called when SelectedIndexChanged event in DropDownList boxes was fired.

In either case, DropDownList had property AutoPostBack="true". Therefore,

Before, when a user selected a different list index from DDL, it won't check validation. So, it worked. (It validated inputs only when a user clicked on Send button to submit this form).

Now, when a user selected a different list index from DDL, it WILL check validation. User can go nowhere since he/she need more data based on the selected item in DDL. So, now it does work.

If I get rid of isValidInput() function, events are fired as normal.

How to ge ...