Sounds like your browser isn't handling the Ajax code properly. Older version of Internet Explorer, perhaps?
As you can see above, the Ajax loads into the center of the screen, and atop the rest of the page.
Some quick guesses -
1. Have you cleared your browser's cache history recently? That is something that needs to happen,
at least once a week. I clear cache daily. And if something acts strange, I will often clear cache multiple times a day.
2. Are you running Internet Explorer as your browser?
2a. If yes, is it the latest version of IE for the version of Windows you are using?
Windows XP users should be running Service Pack 3 and Internet Explorer 8. Bearing in mind mainstream support for XP ended in April 2009.
Vista users should be running Service Pack 2 and Internet Explorer 9. Bearing in mind mainstream support for Vista will end April 2012.
Windows 7 users should be running Service Pack 1 and Internet Explorer 9.
My preference is to not use Internet Explorer at all, actually. My Windows box runs Win 7/SP1 and IE9 for site testing purposes only. If I must use that computer at all, I run Firefox 8.0.1 as my browser of choice.
A quick fix to enable you to log out is to right-click the Log Out link and select open Link in New Tab. That will actually bypass the Ajax code and load in the full Log Out page.
Out of interest, if you try this method, do you see the Log Out dialog appear in the center of the page, as above? Or does it appear in an odd location?