I have Windows XP and am trying to install HP

Hello my friend,
If the HP device will be used locally you will not need to open any port, because open port mean do the port forwarding which is used only in NAT mode but anyway if they ask if the port UDP 427 is open that means that you need to access to the router web interface which is usually 192.168.1.1 and then under Port forwarding you will need to add a rule with external port: 427, Type: UDP, internal port : 427, internal IP: the ip of your HP device. and then save and reboot. everything should be fine after that.
Best Regards,

UDP is a protocol that needs to be allowed to gain access through the router on port 427. This should not be a problem in the default setup on the router. The manual on page 13 (in the one I have) shows the setup for the Security > Firewall portion of the software. There is an Internet Access section that you need to get to. Inside there is a "Services" section that will tell you what services are blocked. You should see a section that describes what policies are enabled and what are blocked. This is where you edit the allowances to add your protocol on the corresponding port. You only need this access from one device to the other on the LAN, not WAN. You also might want to register the MAC address of the Printer in the DHCP pool as a reserved address, that way the printer always gets the same address when it connects to the router. Just setting a static on the printer has the potential to cause you problems, using the MAC ID setup works the best in most configurations. Once you do this, make sure the printer setup on the computer has the correct address. A little more info than was requested, but there ya go!