I had this problem where I had a SMC 2635W 801.11b wireless card that was working perfectly fine. Then I did the Windows XP update to SP2. I couldn't get the wireless card to work anymore. It was finding my wireless router, but something was messed up with the DHCP and it couldn't obtain an IP address. I turned the windows firewall off, tried all the configuration options. Since this was an old product that was no longer supported, SMC did not have new drivers for SP2. I was about to give up and buy a new card.
Then I uninstalled the SMC card driver and re-installed it again. It worked perfectly without any configuration! So if you have a similar problem after update to SP2, give this a try.