When trying to install drivers for a client adapter on a Windows 2000 machine, you get a message saying The form specified is not one known and recognized. This error is security related.
Run the Microsoft Management Console and load the security configuration tool. Select the option that installs unsigned device drivers.
For more information refer Get New Drivers and Firmware section of Cisco Aironet Driver and Firmware FAQ
WLAN adapters (wireless card) / ACU (Aironet Client Utility)