1. Quit GeneXus if it's opened.
2. Remove the file genexusworkspace.xml from "documents and settings" folder (e.g. Win XP: C:\Documents and Settings\UserName\Application Data\Artech\GeneXus\10\genexusworkspace.xml. Windows 7: C:\Users\<user>\AppData\Roaming\Artech\GeneXus\10Ev2\GeneXusWorkspace.xml)
3. Start Genexus.
Tip: using %appdata% with Windows Explorer or command line prompt you can access the folder where the workspace is saved