The Windows version requires the .Net 2.0 framework which is integrated with Windows Vista. Windows XP SP2 users need to download the framework from here and install it prior to MySQL Workbench.
More information about the project can be found on the product page (here) and the MySQL Workbench Developer Central site http://dev.mysql.com/workbench/.
Windows downloads
MySQL Workbench requires Windows XP SP2 with .Net 2.0 or Windows Vista (which includes .Net 2.0).
Visual Studio 2005 SP1 Standard Edition is required to build the Workbench sources on Windows. Apart from the Workbench sources the dependencies need to be downloaded and extracted to the same directory as the sources.