A family member of mine is house sitting and has been given the details of their wifi. The access point is an Airport Express, it has WEP encryption (I think) and they've been given a passphrase to use. I know it's a passphrase and not the encrypted key as it's an English word. The passphrase is 10 characters long.

The problem is that Vista complains that it's not a valid key as it must be a 5 or 13 character non-hex key or a 10 or 26 character hex key. (From what I've read this suggests the encryption is WEP?)

I've found a couple of suggested solutions, but I'm not actually at the house at the moment so I wanted to make sure I have a good chance of getting it to work when I'm there but have no internets to ask.

Solution 1:
Vista needs to be told explicitly what kind of encryption and key is being used. Specify in the connection settings that you are using WEP and that it is a "shared key".

Solution2:
Try converting the passphrase to hexadecimal using an ASCII->hex converter and entering that.