|OK. With WEP there is more to gaining access than just the WEP key.|
There are two other parameters that also have to be correct in order to gain access.
First is the type of key in use. It is probably a Shared key. The second is the key sequence number. ( 1 to 4 )
Since this is in a home environment the person who set up the security should be able to confirm for you whether the key is "shared" and which sequence number is in use. When you have that information you need to go into the advanced settings for your PC's wireless connection and set yours to match those set in the router.
Goin' Fishin' (Some day)