This is a known IE7 bug that kept be busy quite some time a while back.
This starts happening when you introduce position: relative
to one of the elements inside of the container. This is fixed by adding position: relative;
to the container itself, in your case
#slider {
..
position: relative;
..
}
I started developing on a mac as well a year ago, It's a real pain when confronted with IE bugs. It helps if you setup a Remote Desktop Connection to a test PC if you have one and install IE Tester on it (mind though that IE Tester is not 100% the same thing so you should always do a final check on the real thing).
On a side note, Adobe has a really sexy browser screenshot test app at http://labs.adobe.com/technologies/browserlab/