How do I deauthorize ArcGIS Server 10, so that I can do a fresh reinstall using the same license? For desktop there's a "Deauthorize" button for this in the ArcGIS Administrator and in the License Manager, which starts the SoftwareAuthorization.exe file with the -return <license code> parameter. I'm guessing (hoping) there's a similar option for Server?
c:\program files\esri\license\keycodes
You may remove this keycodes file from system after uninstalling arcgis server. This file after changing the extension to .txt can be read in notepad.