I just found out that Javascrip prompt() function on Internet Explorer 7 is disabled by a default setting.
I wonder whether it is that so dangerous for the user if I write a statement like the following:
var a= prompt('What's your name?','') ?
Firefox is not so strict.
Tools -> Internet Options -> Security and then customize the Security Zone (Internet, Local Intranet, Trusted sites, Restricted Sites).
Just enable the radio button to let the scripting language ask info to user via a window dialog.