Vista / XP firewall

I have a new vista premium PC and an XPSP2 machine wired to a speedtouch adsl
modem. the Internet connection is fine on both machines. I have file +
printer sharing enabled and all the other bells and whistles set correctly
.... The Vista machine can see and access the XP machine ONLY when the XP
firewall is off. If I turn it back on - The XP machine dissapears. Vista
diagnoses a likely problem with port 445 on the XP machine. When I try to
amend the XP firewall settings to open this port, I get the message that ''
port 445 already exists SMB over TCP ''

Also, with XP firewall turned off, -i.e. the network functions , when I try
to access the Vista root drive ( which is set to shared everyone ) I get an
access denied, pls contact . . etc etc message ... With the XP firewall
back on, I can still see the vista machine and can access the public folder
only.

Now I'm stuck ! Help anyone ?