FTP server running on Windows XP professional

I have been using a the FTP server running on Windows XP Professional with a
static IP for years with no trouble.
I have recently added a router as a firewall and port forwarded port 80 for
the web server no problem there and port 20 & 21 for the FTP server. I don't
think I have all of the necessary ports open to allow file transfers. I keep
getting permission errors. You can get a connection but transfers fail. Has
anyone set this up before without opening so many ports that the firewall is
useless. Is the version of IIS released with Windows XP Professional capable
of assigning ports for these transfers I understand you need 2 ports per
connection. Any help with this problem would be appreciated.
--
Al