On a windows PC install the printer driver on the required PC.

In printer Properties. (win7 - click Customise Printer (you don't need the Printer attached to the for this) Then Click Ports in there Click Add Port. Select Standard TCP/IP Port, then New Port Wizard starts, Click Next in the next page type in the IP Address of your Router with the Printer attached. (e.g. 192.168.1.1). The second line fills its self in.

Then Click Next and wait use the Drop Down arrow to select “HP JetDirect” then Next and Finished

The Wizards should have added the Port to the Printer

Then all that you need to do is Print off a test page and jobs done