You have really only a few options, XP does have it's own built in firewall that can do the jobby (through the advanced tab in network connections somewhere there) which by the way will be enabled by default with the upcoming service pack 2, uninstalling TCP/IP and client for microsoft networks as well is a possibility but typically you want to either firewall the machine directly or in the case of a network (where the machine must communicate via those ports to other machines on the network) you will want to have some sort of border protection.

A router running NAT in and of itself will typically keep these ports somewhat safe (but remember NAT must never be confused for firewalling) but you do need to remember that most nat providing devices work on the stateful scheme, if your machine initiates a connection inbound traffic to those ports can result. Best bet is to use the built in XP firewall for some basic protection if you dont feel like running anything else.

~~sorry for reviving a bad thread but hey... I got things to say~~