can any of the other computers connect using the wep key? would it be possible for you to try usiung another computer to test this? and the inverse to this is can this laptop connect with wep to another wireless access point?
try using a different wep, make sure you are copying the ENTIRE WEP. IN SP2 there should be a checkbox that says "hide characters as I type" uncheck that and see if you are getting the correct WEP.
make sure you have the latest drivers for your card. Try unitnstalling and reinstalling the drivers. try using an older version of the drivers.