Do some more debugging. Check the value of responseText for example. Check the values of your other variables.
One thing I'll throw out at you, is that IE is more aggressive caching ajax requests than some browsers. Make sure your web server is sending appropriate http headers along with the response, to indicate to the browser not to cache the response. This is the prefered way. The ugly way is to append an ever changing value to the query string for each request, like the time in miliseconds. Well, assuming you don't want it cached.
In any case, clear your cache. The browser might have retained a previous version from while you were developing.