If you are beginning to test Vista, and are looking into the various activation methods, you will quickly learn that KMS activation requires a minimum of 25 PCs. Therefore you may need to begin testing with a MAK key and later switch to activating against a KMS server.
In order to switch back to KMS activation, use the following command lines for Vista Enterprise:
slmgr.vbs /ipk VKK3X-68KWM-X2YGT-QR4M6-4BWMV
slmgr.vbs /skms SERVERNAME
slmgr.vbs /ato
For Vista Business, substitute the key with YFKBB-PQJJV-G996G-VWGXY-2V3X8. See http://technet.microsoft.com/en-us/library/cc303280.aspx for more details on Volume Activation 2.0.
If you need to switch back to a MAK key, simply use these command lines, substituting XXXXX-XXXXX-XXXXX-XXXXX-XXXXX for your MAK key:
slmgr.vbs /ipk XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
slmgr.vbs /ato