open a command or cmd window and try
nslookup regpagename
where regpagename is the name of the registration page that you are trying to get to. What happens? Do you get the correct IP address?
You might check the etc/hosts and etc/lmhosts files to make sure they do not have erroneous info. On a 2k they are located at C:\winnt\system32\drivers\etc
Is the registration page secure by any chance? https instead of http?
Should you be able to ping the gateway? Many do not respond to pings anymore for security reasons.
Did you verify on the switch that the switch thinks they are in the correct VLAN?
Ron