You can use this type to fire Button Onmouse over event...

PageLoad()

In Asp.Net, we can get the mouserOver event for button using Page add attributes...

{

btnExMouseOver.Attributes.Add
("onmouseover", "return validate();");


}

Then just using java script for that function in HTML page
ex:
<script language='javascript'>
function validate()
{
here put your coding....
as per your requirement....
}