Then, there probably isn't a Win7 version driver yet. I wouldn't recommend trying to get a third party driver. Assuming one exists. It will probably cause you more problems then it's worth.
Recommend you either wait until your HP creates a Win7 driver or upgrade your print server to a version that already comes with a Win7 driver.
An outside chance is that the Vista driver might work. It's worth a try ...http://h20000.www2.hp.com/bizsupport/TechSupport/DriverDownload.jsp?lang=en&cc=us&prodNameId=234949&taskId=135&prodTypeId=18972&prodSeriesId=28830&submit.y=7&submit.x=5&lang=en&cc=us
Unless you've already tried that.