u can do it in javascript.
<input type=text name='txt_box' onfocus="hide_but();">
function hide_but()
{
// i assume the id of the button to be 'but' document.all.item("but").style.display="none";
document.all.item("but").style.visibility="hidden";
document.all.item("but").disabled=true;
}
u can use either of the 3 ways used above.
display : removes the control from screen and re-adjusts the rest of the controls.
visibility :hides the control... it occupies space on the screen but is not shown.
disabled : the control is there on screen...visible but user cant click it. it doesnt get focus any more. unless u say disabled=false.
hope that sloves it.