XP Slow

I have several sites that have recently reported slow networks and slow logons. They were ok, but over the last month they seem to have suddenly slowed down.

-Each network is behind a hardware firewall.
-All PCs are XP Pro.
-Servers are 2003, but seems ok speed.
-PCs are running antivirus.

They are not running roaming profiles so logging on shouldnt be slow. They all seem to slow down while applying user profile.

Any ideas