The bane that is IE and CSS

Normally I do all of my sites in flash but I ran into a client who wants to have alot of SEO work done with his site so I opted to use html with SSI. The site looks fine on all the computers at work except one, the site also looks messed up on my home pc. I’ve narrowed down the issue to IE not reading CSS at all. Generally I wouldnt be too concerned if IE is messing up style sheets on some computers but the client’s IE does this also, which reflects poorly upon me. here’s the URL and screenie

http://www.citruswebhosting.com/web-test/index.shtml
http://www.alphabranch.com/screenie.jpg

*as I was writing this post I recieved a call from the client saying that my suggestion to download IE7 fixed the issue. I still would like to know what the issue would be in IE 6 that would cause this. Any info would be greatly appreciated!

-alex