Why are you using WEP -- if you have the choice go with WPA PSK TKIP which is simpler, safer.
Maybe the issue you are having means you have an older version of XP -- install service pack 2 ?
...you've got it the wrong way around. and older, pre service pack 2 build of WinXP should work fine with WEP- its wpa that old versions of windows have problems with.
That being said, yeah, you should just set your router as WPA-PSK if you can. A lot of newer devices-such as the iPad- do not work with wep, period.