The following steps will guide you through an uninstallation of ArcGIS Server for the Microsoft .NET Framework:
Go to Start > Settings > Control Panel > Add or Remove Programs on Windows 2000; on Windows XP go to Start > Control Panel and double-click Add or Remove Programs; on Windows Vista, go to Start > Control Panel (switch to Classic View, if necessary) > Programs and Features; on Windows Server 2008, click Start > Control Panel (switch to Classic View, if necessary) > Programs and Features.
Select ArcGIS Server for the Microsoft .NET Framework from the Programs list.
Select Remove to remove ArcGIS Server .for the Microsoft .NET Framework from this machine. Select Change to install or uninstall specific installation components. See Adding additional installation components for information on adding and removing installation features.
If you do not want to apply your ArcGIS Server configuration (for example, server objects) to your next ArcGIS Server installation, delete the .cfg files located at <ArcGIS Server installation location>\cfg.