Recently I was struggling with accessing my Network Server (Win XP Pro) remotely with Win 7 Pro using PSTools. After setting up an account on both PCs with the same username and password (which is necessary to get remote-control-permissions) as well as turning off simple file charing on my server I still got the Error-Message "Access denied". This problem was caused by an Win 7 Update which changed a Network Security Option.
Here's how u rearrange the compatibility between Win XP and Win 7(/Vista).
Vista and Buffalo Terastation (English)
Windows 7 Systemfehler 86 Netzwerkkennwort falsch (German)
good luck! ,)