Site not showing properly in some browsers

I recently added a horizontal menu to the top of a client’s site, and it doesn’t appear to be displaying correctly for some people. Originally, I saw the problem as well, on my computer on Firefox. Instead of being a horizontal menu, it was showing to me as a long vertical menu on the left, messing up the top of the page. But then I emptied cache and fiddled with the menu, resaving it and some combination things I did must have worked because it started showing me the correct menu.

Now, my client has still never been able to get her browswer to show her the right menu! She is seeing the vertical menu in Chrome, but it looks fine to her in IE.

I have polled about 5 or 6 of friends and every single one is seeing it correctly on their desktops. The website is http://www.gymnasticsbeat.com And I imagine it’s relevant that I used Artisteer to create this site and add the menu.

What do I do now? Is it a cache issue for my client? I told her to empty her cache, so I assume she has. How do you fix a problem like this when I cannot get my computer to see what hers is seeing?

I’m no professional web designer…I’m entirely self taught and learning this as I go, so any experienced people that have insights I would really appreciate hearing from!