Let's say you've built a new workstation and it is using an MAK but you want to switch it back to being a KMS client. The guidance is here:
But what is <setup key>? Should you put your KMS key in the workstation? NO! Not unless you want it to be a KMS host and utilize one of your KMS host activations. Another part of the VA step by step guide explains what key to use. Look on the same page under "Product Key Deployment Considerations". The <setup key> to use with slmgr.vbs is stored on your Vista media under Sources\PID.txt. This key will tell the workstation to be a KMS client.
The Trusted Platform Module is not a Microsoft technology but Vista leverages TPM as a preferred hardware component when enabling BitLocker drive encryption. The Trusted Computing Group maintains the specifications on TPM and provides a great deal of technical background information.
Link to Trusted Computing Group: TPM