Adding 4th XP PC to Home Network

For some reason my 4th PC won't work with the flash drive when it comes to
adding it to my home wireless network.

I have the printout which shows the Network Key etc, but I can't for the
life of me work out where I put it (so to speak)

I have a Draytek Vigor 2600Gi wireless router. My 4th PC can surf the
internet via the router, but when I hit view wireless networks it doesn't
show it.

When I tried to set it up using a flash drive I got a warning message to say
it probably wouldn't work.

Help!

The PC I'm trying to set up seems to need a 13 character key, but the key I
have printed out from one of the other computers is 26 characters long?