Rather vague question but:
A port will not be acsessable unless there is a service utilizing it, or you have a firewall in place. XP has a firewall in-built you may wish to check, under network properties,
Don't put too much faith in Shields Up either, I have had varying results from the same computer within only a few minutes. It also only checks about 8 ports out of the thousands you're computer is running, so if you want peace of mind make use of a firewall on your machine.