Disable your router security and see if you can connect. If you can, I'll wager it's a HEX vs ASCII issue you're having. Go here
and generate a WEP key - you'll have access to the ASCII and HEX versions. Copy both to a text file, then enable WEP again and use the new generated key. If the ASCII doesn't work on the computer, try the HEX version.
But, in all honesty, you should use WPA2 with AES, if your hardware supports it. WEP is a very weak form of protection, and it isn't really recommended any more.