(01) Is the Vista computer able to access ANY shared resources at all on the XP machine?
(02) Are you running any firewall software (other than the firewall built into the operating system) on either computer?
(03) If you are, please, let us know which firewall software it is and, for now, please, disable it.
(04) Do both these computers have Internet access? (I'm assuming they do but I do need to ask.)
(05) Is the printer currently ONLINE and working properly w/ the XP machine?
IF YOU HAVEN'T DONE SO ALREADY, PLEASE, DO THE FOLLOWING:
(06) Assign *both* computers to the same network WORKGROUP.
(07) Ensure that file and print sharing are enabled on the XP machine. If you'd like, do so on the Vista system, too.
(A) How to on Windows XP
(B) How to on Windows Vista
(08) "Share" the printer
on the XP machine (in printer properties).
(09) Can each computer now see itself AND the other computer on the network?
(10) Have you added your user name and password that you're using on the Vista system to the users list on the XP system?
(11) If you haven't done so already, please, do this. The user name should have a password associated w/ it. So, if you aren't using a password on your Vista system profile, assign one then add that user's credentials to the users list on the XP system.
Please, post back here w/ what you've tried and the results, your observations and any other comments or questions.