Windows Vista 32bit
Other versions of Windows should work but are untested.
x64 support is under development (available only inside of Microsoft at this time).

.NET Framework 2.0
Also supported are .NET Framework 3.0 and 3.5.

Pex comes with a Visual Studio Add-in which only works with Visual Studio 2008 Professional (or higher).
Pex also works from the command line without any Visual Studio on the machine.

v0.5.30611.1, 06/17/2008, download pex.30611.1-X86.msi, release notes

Pex is licensed under MSR-LA, (Microsoft Research License Agreement, Non-Commercial Use Only).