The Win32 FreeRTOS simulator was kindly provided by Dushara Jayasinghe. I have used it with Visual Studio 2008 Express Edition (which can be downloaded for free) under WinXP and found it to be a very valuable tool.
See the page on the main FreeRTOS site for more information. The files are attached to this post.
[EDIT Richard Barry] NOTE: FreeRTOS V6.1.1 and later includes two official Win32 simulator ports and demo projects. One that targets the (free) Visual Studio Express 2010, and another that targets the (also free) MingW (GCC) compiler and Eclipse IDE.[/EDIT]