Vista and WinXP won't play nice

Right. I've got a Vista machine that will only talk to an printer shared by a WinXP machine at random times.

Acer Aspire 5100 running Vista Home Premium, no-name running WinXP Pro which is sharing the HPDeskjet printer. All systems patched up.

Also running: Panda antivirus. No difference if I enable/disable this software on either machine. No difference if I enable/disable Windows or Panda firewalls.

Sometimes I can print a test page on the printer. Sometimes I can't. Sometimes it works after a reboot, sometimes it doesn't. Sometimes I can delete and re-add the printer, sometimes I can't. I haven't found a particular pattern yet to the working/not-working saga. Latest HPDeskjet drivers installed on both systems. Also tried with default Vista drivers - no difference.

Anyone have a magic incantation that gets Vista to "just work right", instead of being the steaming pile of :crap it's been so far?