Desktop Grids harvest the computing power of idle desktop computers whether these are volunteer or deployed at an institution. Allowing foreign applications to run on these resources requires the sender of the application to be trusted, but trust in goodwill is never enough. An efficient solution is to provide a secure isolated execution environment, which does not constrain any additional burdens neither on administrators nor on users. Currently Desktop Grids do not provide such facility. In this report we describe our approach to provide a platform independent and transparent sandbox mechanism for Desktop Grids. We define the requirements for the transparency and present a prototype that fulfills these criteria.