Generally you don't "uninstall" an operating system - in most cases it's not just a piece of software handled by some software management program. There are exceptions, like the Ubuntu installer for Windows (which does let you install it under Windows and remove it afterwards, without partitioning), but in this case we're talking about "normal" operating systems.
When you remove an operating system, you simply delete the data and maybe modify your partition layout. If the operating system to be removed contains your bootloader (say you installed Grub along with Fedora, overwriting Windows bootloader in MBR), then you need to install another bootloader to be able to boot any other operating system. This all can be done in a new operating system installation, so if you put the setup disc in, you can install the OS over your previous one like mentioned in the above post by billymayday. That should also take care of the bootloader installation/configuration, so it's not a problem. If you're just willing to get rid of an operating system like Fedora, and want to "get back" to some other operating system like Windows, you need to
- make sure you have a bootloader installed that can boot the OS that stays - typically means reinstalling the bootloader used by the OS that stays
- erase the data on the partitions used by the OS to be removed, if you don't need them anymore; that means formatting the partitions if you want to keep them, or just deleting and re-creating (and then formatting) them if you want a different partition layout - resizing partitions might also be a good idea.