Hmmm doesn't sound good about the school. Most of them block or filter p2p traffic. It is possible your school may have started doing this. I would suggest you contact your school network administrator and find out.

Also WindowsXP sp2 contains a firewall. Have you configured this to allow LW full access. This has to be done for both TCP and UDP connections.

Maybe one of the other members or Mods. might have some other suggestions or ideas also.