You need to enable "Network Discovery"
To do this:
click Start, right-click Network, and then click Properties.
Then turn on "Network Discovery"

By default vista sets all file and folder sharing as VERY secure... this can be annoying as im sure you have just found when first trying to enable file sharing... all you have to do is go into the network sharing window in vista and change all the default values to whatever suits your network.. for example, you will find as default that you need a local profile set up on the vista machine to access it from the xp machine, this can be disabled... this is one of many features that you will need to look at to determine what needs to be set to on or off...
Also check this link. Scroll down to find
Network Access Protection