Every release of Internet Explorer these days seems to cause compatibility problems with existing websites.
Always check your site works properly in the latest version of all browsers… Internet Explorer, FireFox, Safari and Chrome should cover you.
In particular it appears that Internet Explorer 9 breaks a lot of 'dynamic code' used for pop-up windows, particularly in Content Management Systems' admin areas. But don't panic, it doesn't mean that you need a new website! Try putting the following line in every page's < head > section (you might need to ask your developer to help you here, but it's only a 15 minute job):
< meta http-equiv=”X-UA-Compatible” content=”IE=8” / >
and in most cases that should sort you out…