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ReactOS is a free, open source clone of Windows aiming for compatibility with Windows XP/2003, along with some extensions from later versions. ReactOS has been in development for a long time and has not yet reached beta stage, but Lazarus can run on it.
If you want to experiment with ReactOS, it's a good idea to use a Virtual Machine (e.g. VirtualBox) with a snapshot boot CD as ReactOS development is quick and a lot of bugs get fixes between binary releases. It is easy to upgrade an existing VM install by running an upgrade from a newer boot CD.
Because Lazarus can be installed without problems, presumably the same applies for installing FPC. fpcup is periodically tried on ReactOS as well (it requires installing an SVN client first); without success at least up to ReactOS r63093.
ReactOS aims to emulate Windows; it is possible to just run the Lazarus installer. However, ReactOS includes a software installation program (RAPPS) which allows you to download the normal Lazarus installer and install it with a few mouse clicks.
ReactOS bugs are tracked in their bugtracker, e.g. this bug about flipped icons when running Lazarus: