If you get the message "bad param error -4" when you try to administer your AirPort Express, don't worry--the latest AirPort software update may fix that. And if it doesn't, there's an easy workaround.
The issue occurs when using AirPort Admin Utility 4.0 for Windows or AirPort Express Assistant 1.0 on a computer with "Intel Centrino mobile technology" wireless capabilities. To resolve the issue:
First, download AirPort 4.1 or later. Next, try checking the Network Connection that you use to connect to the AirPort Express (usually this is your wireless connection) to ensure that it is configured via DHCP; if it is set for a static IP on a different subnet from the AirPort Express, this issue will occur.
If your issue is still unresolved, use this workaround:
Important: Information about products not manufactured by Apple is provided for information purposes only, and does not constitute Apple's recommendation or endorsement. Please contact the vendor for additional information.