Thanks for showing an interest!
The only APIs it uses are LoadLibrary, GetProcAddress, GetSystemDirectory and a couple CRT functions, so I think it should be pretty backwards compatible, though I haven't tested them on anything prior to Windows 7. Other than that, you might have to adjust the project settings to target older systems, I'm not really sure.
On a side note, I've noticed the x64 versions can be a little unreliable (have experienced non-consistent crashes using the winmm.dll proxy with a game I was reversing) that could be due to stack management issues in the assembly. But the x86 builds should be stable.