Is it a network card or the one which is onboard?
Try the below from the RUN command on Windows - START.
regsvr32 netshell.dll
regsvr32 netcfgx.dll
regsvr32 netman.dll
Reboot the computer.
If still persists, do the below from START - RUN - CMD
Reset TCP/IP stack to installation defaults, type: netsh int ip reset reset.log
Reset WINSOCK entries to installation defaults, type: netsh winsock reset catalog
If that doesn't help, try this small utility . .
http://majorgeeks.com/XP_TCPIP_Repair_d4521.html
Also check if after you install SP3 you had this issue, if yes try to unistall it for a test and see if it works fine.
Hope this help!
Kev