Of course here's more than 2 way to backup the windows. But first I need to know which windows you'r using ? If you using Windows 7 or 8 its very easy to backup.
For windows 8 you can check there:
http://arstechnica.com/information-technology/2012/12/using-windows-8s-hidden-backup-to-clone-and-recover-your-whole-pc/
and for windows 7 you can check there:
www.youtube.com/watch?v=rS616jQOURg
the two process is almost same and very easy, just require few steps to complete,
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If you want just partition your HDD that contains the Windows OS but you don't want to make change on the OS drive then you can do partition very easily.
Steps are below (for only windows 7 & 8 dont know about earlier versions):
1_ Go to "Control Panel" and type "disc" then click on "Create and format hard disk partitions" under "Administrative Tools"
2_ Then click on any drive and you can delete/shrink/expand & create new volume. you also can change drives letters here.
This all applicable for the drive that not contains the OS. Its no Problem if you'r OS drive stays in same physical drive. This will not delete the Windows OS. But Make sure to backup the drives data to another drive of the same physical drive or somewhere else before deleting/formatting the specific drive.
But if you want to do this by Windows XP this may be different and may you need to backup hole HDD data to another physical drive.
Hope this will help you.
Best of luck :)