You have to buy the MAC operating system and then install it on the computer.
You cannot run Mac on Windows XP. But you can run Windows on Mac
There are certain tricks to install MAC on PC (or Windows Xp on MAC) but it's rather challange and requires special skills. Thus, the answer is NO, MAC is not compatible with Windows XP.
You can install Windows XP on your Mac with pentium-3.
Either Windows XP or Vista can be installed on a Mac that uses an Intel processor - which is all of the recent ones.
Yes, there are Mac themes for Windows XP available at RocketDock, the blog XPMac, Softonic, How to Geek and Mac Update as well as many other places on the web.
No unless the Mac is running Windows XP or Vista as there is no Mac OS X version.
Yes. Install OSX first, the use Bootcamp to install XP.
Yes, you need an Intel based Mac.
On an Intel mac you can run parallels which is capable of running xp and vista.
I generally prefer Windows XP, but sadly this came out of support earlier this year so we all have to use Windows 7, 8 or Mac osx :)
Microsoft Windows (XP, Vista,7) and Mac (Mac OS, Apple)
There is no rollback way to do it. You would need to reinstall XP fresh over the Vista install. Why do you want to do that anyway, is Vista not working for you?