Daniel, I'm unclear on what you mean by having your XP machine set up as a printer server. Are you simple sharing printers on it, or using some other software/hardware?

If the printer is shared, and the Win7 machine can't detect it, you can try installing it as a local printer to LPT1, installing a driver manually, and then changing the port. Go to Devices and Printers, add a printer, then choose the local printer option on LPT1. Click next, then when you're prompted to select a printer, you'll need to click the Windows Update button because the 4050 won't appear as a choice in the list of HP printers otherwise. After the list is refreshed, click on HP in the left pane, and scroll down the list until you get to the LJ 4050. You have a choice of a PCL 5 or 6 driver. Pick one, then install it.

When that's done, go to the printer properties menu and change the port to a new local port named \\xpcomputername\printer name , apply the changes, and see if it prints.