Help connectivity problem

Am trying to install a Cisco catalyst express 500 switch in place of a hub. 6 Windows XP machines connect to our head office and internet through the existing hub. When I put this new switch in its place 4 do and 2 don’t. I’ve tried using different ports always with the same 2 not connecting. All 6 are the same Windows XP machines all have an IP address assigned automatically.

Any help and suggestions gratefully received.

TIA