>We are using the mac address to identify the client machines on a network.
>However now that they are using some Windows 7 terminals, and they are using some wireless device, it seems that those machines do not get a mac address. At least our code does not return any value.
>What is the second best way to identify a computer, the only important thing is that this computer must generate the same key each time and it must be unique.

What may be happening is the workstations have both real (wired or wireless) and pseudo network interfaces. Each of those has a MAC address, so saying you are using "the" MAC address is not enough, you will probably need to enumerate them and determine if any is the one you want e.g. http://www.tek-tips.com/viewthread.cfm?qid=756938

There are other hardware options in that link. But, one thing you might want to consider is the host's NetBIOS name, which must be unique on its network and is a PITA to change. It's available via SYS( 0 ). In contrast it's not hard to change a MAC address (although duplicate ones on the same network causes problems too).