Well what I have found is not an OS, it is a theme for windows that transform it to look like its running mac OS X.
If you want to try it here what you need to do.
1. Go to this site and download the app. It dose not cost.
2. After downloading Tiger V. copy theTigerV folders and TigerV .theme files to XP's default theme directory, To get to the Theme folder; click My computer then, C: Local Disk, Then Windows, then find a folder that says Resources. The Theme folder is in there.
3. After copying the files, right click on the desktop and click Properties. Under the themes tab you should find Tiger V click it.
4. Now you will notice that you have a Mac like theme only one more thing.
go to this site
and download that app.
5. After downloading launch the app.
I think it dose a good job mocking the Mac interface.
What do you think