Do you have any electronic equipment right next to the cabling. Such as Cell Phones, motors, anything that might interfere witht the line. I know you might not believe it but stuff like that can slow a network down. Also, are you using cat 5 cable or using something different.?

I can't really think of a security setting in XP other than maybe disabling QOS(quality of Service) Packet Scheduler. To do that go into network connections then right click on your lan connection. Select the networking tab and where it says this connection uses the following items, uncheck the QOS packet schedule.

Thats all I can think of right now.