you can follow the following steps:
Step 1:get the ip address of both the computers
make sure they are in the same work group and has different computer name.
step 2:make sure they are able to ping each other , if not disable the firewall
step 3:share the drive or the folder that you wants to share in vista, by right clicking on it and click on properties and allow permission for everyone.
step 4:type in the shared name
step 5:you can disable password protected in vista computer.
step 6:from the xp computer go to network->view work group computers and click on the vista computer , you should be able to see the shared folder or drive.
all the best.