SBS 2003, Internet Access to Clients works only on some pages.

I am running SBS 2003 at a non-profit org with 5 clients. I have 2 nics in the server with one going to the modem, the other to the domain, with a wireless access point and switch. My clients can access most websites, like this one, google, and yahoo, just to name a few. But, I can not for the life of me log into MSN’s network, or a student loan site, or myspace. While these are not problems for Corps who typicaly ban these sites, I need them to run our communtity services dept. Any help would be very much appreciated.