How do i change my operating system on my computer?

OK someone has downloaded a earlier version of windows to my computer which is windowsNT,originally i had windows vista home premium, I really need to know how to get my computer back to the original version.How do i change my operating system on my computer?
You need to reload or reinstall again the Windows Vista OS. All you need to do is to insert the disc, then follow the prompts.How do i change my operating system on my computer?
Unless you have the operating system CD, you have 2 choices:

1. Throw money at the problem by buying another Windows CD. Cost: $200-$399.

2. Get a much better open source operating system at www.kubuntu.org, for free. Get Feisty Fawn, at that site.How do i change my operating system on my computer?
you can't just simply download an operating system onto your computer, it would have needed to be formatted and then NT could have been installed. More likely what happened, someone just changed the visual appearance of the desktop, or someone is playing a mean trick with an emulator