My first usefull ajax script!

First you want to remove the onclick from the button.

Add this to the form:
onsubmit="GetURL(); return false;"

The return false will make the form not submit the page, you know what the GetURL function does.

There error in IE is this line (which IE doesn't support):
Request.overrideMimeType('text/html');

To be honest, I'm not sure why you're using that. Your returned result should send back those proper headers if you must have them in there. If you remove that line it looks like it should work.