There are numerous threads in the forum as well as several FAQ entries on the subject of getting Windows and the DAAP to play well together. Chief among them is to ensure that the built-in Windows firewall is letting TCP port 3689 and UDP port 5353 packets through. If you are "seeing" the share, but cannot play anything, it is likely the UDP port which is being blocked.
Check those settings and let us know if corerction solves your problem.
If the WinXP box is properly configured, check any router which may be between them, especially if the Roku and the MCE hardware use different networking protocols (e.g. one is hardwired and the other uses 802.11x).