First, I would suggest installing some sort of antivirus program on the subject machine; any brand/any flavor, just put something on and run a full scan with updated definitions/signature files with the machine removed from the network.
If no firewall is installed, install one; do not rely upon WinXP or WinXPSP2 to supply the firewall, as neither has egress (outgoing) protection. There are several good free products out there, such as Sygate (my personal choice), Zone Alarm, Black Ice Defender, and Tiny Personal firewall (there are most assuredly others as well).
With that done, check and see if there is an entry in Device Manager called “1394 Networking”: this is an service which has been known to interfere with network (ie: broadband) connectivity. Adaptec knowledgebase article 7797 describes the issue and has a possible fix, especially as relates to their IEEE 1394 products (the files specifically mentioned are Windows XP native files, not Adaptec’s).
It’s also possible that the network card/subsystem may be failing…
Hope this helps