When you say you've changed nothing other than enter your mac address, do I assume you are using some form of mac filter?
If so, have you managed to filter out the mac address for the PDA and deny access?
I would revert your network to no security, and remove all filtering rules: then it should be a simple case of adding the network settings into each device as you add them.
Only then build up security. You shouldn't need to enter mac addresses manually for basic connectivity. Set the router up to assign ip addresses automatically and only then try to change things.
It could be the PC adapter failing, but this should not affect other devices.
As for slower speeds - the more devices you add, the slower the speed for all on the network, because the internet connection is shared between the devices connected.