<form onkeypress="checkKey()">
    <SCRIPT>
    var ctlSelected;
    
    function checkKey() {
        if (window.event.keyCode !=  13 )
            return;      
                
        switch (ctlSelected.id) {
            case "textbox1":
                document.all.button1.click();
                break;
            case "textbox2":            
                document.all.button2.click();
                break;
        }    
    }
    </SCRIPT>
    <input type="text" id="textbox1" onfocus="ctlSelected = this;" /> 
    <input type="button" id="button1" value="button1" onclick="alert('button1');" />
    <br>
    <br>
    <input type="text" id="textbox2" onfocus="ctlSelected = this;" /> 
    <input type="button" id="button2" value="button2" onclick="alert('button2');" />
</form>