im not really following you clearly
is the router set to wpa and then reseetting to wep? or is the computer trying to use wpa to connect to the router but the computer still reverts to wep?
if the computer connecting is the issue:
type your password in one key at a time, like a 4 year old. key mismatches are easy to do and happen often.
also, update your wireless drivers on your pc.
if the router is the issue:
backup the config in the administration area, upgrade the firmware.