Yes, you need to use JavaScript. Use a line similar to the following:

TextBox2.Attributes.Add("onKeyPress", "return (event.keyCode>=65 &&
event.keyCode<=90);")

This code returns true if the character entered is a capital letter, and
false otherwise. If the returned value is false, the character is not
displayed. (NOTE: you may want to test this on all browsers, because I am
not sure if it lets the user use the backspace on all browsers, but I know
it lets you on IE6, I haven't had time to look into it yet)