I've found a way to connect with a computer that runs on Vista to a PRINTER who is attached to an XP computer by USB or Firewire.
In my case I used Windows Live OneCare and there you can go to the firewall setings and you sould allow media servers.
Don't FORGET that the 2 computers must be in the same workgroup or domain.
Then you can simpely go to the network connections and the computer should be there. (When it is turned on)
When you can see it just go to control panel -> printers -> New printer
and let him search for it (this could TAKE a wile)
Maybe this allso works with other firewalls but it defenetly doesn't work with the onewho's build in.
If you know a firewall where it works to let me know.

Jonas Thats awsome i have been struggling with this on my home setup i will be giving this a try tonight!If you NEED a free firewall PC Tools has one that works on both Vista and XP. (Freeware)
You might give it a try.
Let me know because I did not try this firewall.
You might also check the protocols of the firewall to see if network detection is allowed.
Let me know if it works with the PC tools firewall. (that would be a much cheaper solution)

Jonas If this was the solution where you were looking for please let me know.
I'm just wondered how much people that actually have this problem.
Oh and if the PC tools firewall works to than I'm really interested.
Please post if it helped you.

Jonas