Wordpress issue with permalinks and IE?

Switched my site to Wordpress recently, and had a designer customize a template for us…but now I can’t change the URLs to standard naming conventions without breaking the code in IE. It works fine in Firefox, but when I go to the subpages in the second tier nav like here http://netvantagemarketing.com/?page_id=191, the style sheet doesn’t load and I end up with a bunch of jumbled text and images. I really would like to go by %pagename% for obvious reasons, but I can’t seem to get past this issue in IE. Any suggestions?