Problems with security settings on wireless broadband

[#1]

Hi
I'm a newbie to broadband.  I've just started a plusnet account and it's going fine except for one interesting problem.  I have one pc connected by ethernet cable and two pcs connected wirelessly.  Normally I have the security setting in the wireless access point option on the router's webpage set to 'new devices are allowed' and the two wireless pcs connect no problem.  However, if I set the security setting to new devices are allowed by registration' or 'not allowed' then the instant I do this i Iose the connection on one of the pcs but the other stays connected.  It doesn't matter which one or if both pcs are switched on, the desktop with a wireless card will loose the connection as soon as I change it to anything other than 'allowed automatically' and will get it back when it's changed back but the laptop doesn't seem bothered what I change it to.  I've obviously got a setting wrong somewhere, both pcs appear in the list of pcs under the wireless logo in Home Network
Any help appreciated
Thanks - Steve

[#2]

Weird.
However, if you have WPA security and you aren't expecting anyone else to have physical access to your router, why not say allow access to all and sundry?
if you only have very weak security, then perhaps it might make some difference.