Upgrade from 2000 to XP

I have a small home network running Windows Server
2003 with 4 clients.
All the clients had 2000 on them and all was well before
I added a new PC and upgraded 1 to XP Pro, now when the
computers sit for a couple of hours
the XP PC's run very slow, the 2000's work fine. It takes
for ever for MY
Computer to open and if I start something that has data
on the server it
takes time, but if I restart they work fine. I ping the
server and it works
great and the ping is fast. The other odd thing is that
if I let Outlook
open and running minimized and the auto send and receive
mail is going the
system does not slow down, why?

What is different from 2000 to XP that I am missing?
The server has a
static IP too.



Thanks

Joe