Wireless Network Help

I am trying to share my ethernet connection that my mac has with a vista PC over airport (and a wireless adapter in the PC).

It connects completely fine when I have no security enabled on the airport network, but as soon as I add a 5-character WEP key, the vista machine can't connect.

I've used 40-bit encryption and the software on vista only gives me option for 64 bits and 128 bits - could this be the problem?

Please help

EDIT: This is being set-up at university so I don't really want to leave my macbook with no security on it

Nevermind, I have sorted it now.

I found a WEP convertor which gives me the key in numbers, entered that instead and it works fine

Peace