I've been having similar problems.
Tried the netsh to reset TCP/IP ... no luck.

Came upon some other information that looks promising. Other forums have reports of, with XP, recent TCP/IP and WINSOCK stack corruption. This appears to either be caused by spyware or simply the fact that since TCP/IP is now deemed a critical 'core' networking component of XP, its a little more fragile than in previous Win O/S versions.

Following are a couple of tools to try ... one to repair the TCP/IP stack, the other for WINSOCK. I have not tried these yet ... but I intend too soon. These tools have apparently aided others in getting this %$&@ problem solved.

TCP/IP - http://www.cexx.org/lspfix.htm
WINSOCK - http://tntmax.com/Download/Software/WinsockXPFix.exe

Best of luck.