hiya MaryAnn,
to share files you need to navigate to the folder you wish to make available eg my music, right-click on that then click on properties then you go to the sharing tab and put a tick in "share this folder on the network" then Apply, the folder icon should then change to a folder with a hand underneath.
A word of warning though any sub folders and files contained within the shared folder will also be available to any other computer on your network.
On the laptop you should then (assuming firewalls are not getting in the way) be able navigate to workgroup computers from the laptop and see the xp machine if you click on that the folder you've allowed sharing on should then appear and you're good to go.
It is a little more convoluted to share vista files with another computer the right permissions need to be set after telling vista to share the file. but if I read your post correctly you want to share from your xp machine to the vista machine.
hope this helps, Gary