This kind of problem is often down to one of two things:
1) the WEP key is wrong - double double check this
2) old or multiple instances of the driver conflicting with eachother. Your comment that you got your old installatiopn of XP working on a new motherboard makes me suspect this. Try completely uninstalling the wireless card and all its drivers, the whole lot, then installing it again from scratch. This might work. Once I had a system that was so confused the only cure was to reinstall XP after which the wireless worked first time
Thanks for that Bishop - I did the usual uninstall drivers, remove from device manager etc. then tried both allowing windows to find and try and install the device as well as installing using the manufacturers driver software.
Regarding the WEP - it's a Sky router and I've not gotten around to contacting Sky for the router admin details yet so it's the code that came on the 'put in a safe place' card.
You could try logging into the netgear via cable and turning off the encryption (only for testing) then at least you will know if it's an encryption issue or hardware issue. Also check the wireless connection properies just incase an incorrect WEP key has been saved to the connection props.