There are 2 ways to stop XP from sniffing for other APs:
Manually type in after connecting to the net :
NET STOP WZCSVC
or create a .BAT file that you place on your STARTUP folder:
:LOOP
ping
www.yahoo.com
if not errorlevel 0 goto LOOP
NET STOP WZCSVC
Your PC might loop a couple of times before establishing connectivity. If there is a better way, please let me know.