Wordpress Custom Theme CSS Fix - Very Small/Fast Job

Estimated duration: 30 minutes
Description: Hi

I am a designer and have never been a coder have dabbled here and there. Anyways sort of made a custom Wordpress theme and I was happily plodding along learning everything on the way on a test installation of Wordpress 3.6.

Have ran into a problem which I have spent 5 hours trying to fix. I have a test navigation menu on the site and whilst in renders fine in Firefox and Chrome its out by about 10px in IE10.

So looking someone to tell me what I have done wrong and how to fix it please.

Sorry its not a huge budget but I imagine for the rest person its a 5 minute fix.

Site is www.nisites.com/wordpress (please ignore everything else as I say its a test site just want help with the button out of whack issue)

Thanks