WPA or WEP Security Question

[rocksolid]

Need some help with wireless security...

My router has security options as follows:

WPA or WEP
WPA only
WEP only
None

It's one of those pretty much configured out the box by my ISP jobs and the default is 'WPA or WEP'. I want to change this to WEP only as i've just got a Nintendo DS and it doesn't support anything other than WEP.

When i do this though i can't get connected wirelessly at all. Anybody got any ideas why not? I don't think my laptops have any WPA settings, all i know is i had to input the WEP key to connect them to the router, don't know anything about WPA settings.

Any suggestions appreciated