First, there are some updates to Win XP software not normally automatically installed via automatic update which are needed to support the enhancements to TCP/IP in use in the Vista environment.
Second, due to Vista operating systems requirements, all accounts which may access anything on a vista machine are required to be password protected in some way. This means that a Windows XP Home user without a password on system A can never "see" anything on a networked Vista machine even if all the file and printer sharing is set up correctly.
Third, some security software can give you problems. For example, on my little home network, two PC's are running Symantec's Internet Security, and a third has McAffee downloaded for free from my cable service provider. The McAffee software equipped machine is denied access to the Semantec equipped machines not by Symantec, but by the design of McAffee's own product. It is a PITA, but all of your poblems are solvable.