XP cannot access Vista how? Through Remote Desktop? If that's the case, you need to update your Remote Desktop client on Windows XP and also go to the System Properties --> Remote tab, and choose the second remote desktop option in Windows Vista, since XP does not support the extra security that Vista does natively, just yet.
Perhaps this is what you're looking for, a link to a download that will update your Remote Desktop Client on Windows XP SP2:
http://www.microsoft.com/downloads/details...;displaylang=enHope this helps!