Windows XP FTP Server time out errors with FTP clients

I am a network administrator & computer consultant and my background
on this topic is good. I was runing serv-u FTP server on windows 2000
behind a linksys router for years without any problems. Just upgraded
to Windows XP Pro. FTP server will not work with certain FTP clients
mainly internet explorer's FTP client. I have made sure it is not
windows xp firewall, i have set it up correctly. I have even disabled
windows xp firewall. The problem is not with my linksys router, this
has worked for years, a simple port forwarding to my computer's IP has
alredy been setup. I changed my ftp server thinking it was
incompatible with windows XP, downloaded cerberus ftp server, same
problem. I think it is something with windows xp and possible SP2,
maybe something with the damn winsock.dll who knows? I have tried
multiple ftp server with no luck, my linksys router is setup
correctly, windows firewall is not the issue. My FTP server shows a
connection from the client but when they run a dir or any ftp command
the connection times out (and yes the timeout on my ftp server is set
to 100 seconds, more than enough time) The only thing i ccan not rule
out is the change from windows 2000 to windows xp pro sp2. One more
thing i noticed also is when i turn off "passive mode support" in
internet explorer advanced options that it seems to work then. I
assume internet explorer would try to directly connect to my ftp
server first then use passive mode. a direct connection will work
because i have port 21 forwarded directly to my ftp server computer
inside my network, this has worked for years before the upgrade to
windows xp. If anyone knows of any information that would help, i'd
really appreciate it. I have read of others having this or very
similar FTP problems as well. thanx