Uninstalling W-8 and going back to W-XP.

Hi everyone,

I'm currently running a Desktop with W-8 Pro and have decided to uninstall it and go back to W-XP or even try W-7.
The Hard-Drive has no other Programs on it.
Can an OS be uninstalled like any other Program ?. I've installed many in my time, but not an OS.
How could this be done with W-8 ?.

I've also been told uninstalling any OS is unecessary, as new Program can be installed over the old one.
Is this correct ?.

Would an actual uninstallation be better ?. What are 'Partitions' and are they important ??.
I have the W-XP Installation Disc and Product Key. Is there anything else that has to be done ?.
Please be detailed.

Is W-7 better than W-8 or XP ?. Why ?.

Hoping someone can help.

T.