At some point in your Citrix experience, you may decide that you need to uninstall and reinstall Xen App or Presentation Server. If and when this happens you will likely need to force the uninstall. Luckily, the mps.msi comes with an operator to CTX_MF_FORCE_SUBSYSTEM_UNINSTALL which can be set to use, rather than hunting through the registry and manually removing entries there. You run the msi through msiexec, as follows:
msiexec /x mps.msi /L*v c:\ctxuninstall.log CTX_MF_FORCE_SUBSYSTEM_UNINSTALL=Yes
Once uninstalled, you can install anew.