If the X-UA-Compatible isn't set in the layout.master or on the website, the only other place I can think it would be set it is the server itself. From within IIS Manager, click the server in the Connections pane. We want the server here, not the website. From there, double-click the "HTTP Response Headers" item under the IIS section. If there is a header listed named X-UA-Compatible, that is the problem. If the site in question is the only one on the server, you can remove the X-UA-Compatible header. If there are other sites, add this to your layout.master:

<meta http-equiv="X-UA-Compatible" content="IE=edge" >

If there isn't a response header named X-UA-Compatible setup on the server, I can't help but think that your layout.master contains the meta switch. If you can't find it anywhere but your site is still coming up in compatibility mode on other machines, add the meta tag above to your header. That should force IE to use the highest mode available.

You can read more about how this stuff works here.

HTH,
Joe D.