client connection to exchange with RPC over HTTP

I have about 40 Users connecting to my exchange server (2003) remotely using RPC over HTTP. I now have 5 clients that are unable to connect. I have not a lot of knowledge on this and have run out of things to try.
1-Reinstalled outlook 03
2-reinstalled certificate
3-made sure the certificate was in the right store
4-Was able to configure the account on another laptop so it is the 5 laptops

any suggestion would be greatly appreciated.