I've had this problem as well, Z3100ps, XP Pro, printer wouldn 't wake up.I called HP tech support and it was an issue with the network setting of the printer.
What he had me do:
1) Under Printer and Faxes, right click on the Z3100 and select Properties
2) Select the 3rd Tab, "Ports"
3) Select "Add a Port" then Select Standard TCP/IP, click add port
4) A wizard comes up for the TCP/IP port. You'll need to know what the TCP/IP address is of your printer, easily seen on the printer's front panel.
5) Follow the wizard.
It's cured my problem.