You don't have to do anything you don't want to. The operating systems are, from the point of view of another operating system, just data on disk so they don't "interfere" apart from bootloader. If you want to replace Fedora with Ubuntu, just install Ubuntu over it -- in the partitioning tool select the Fedora partition(s) to be used for Ubuntu. As long as you don't touch (format, delete, ...) the Windows (or other OS) partitions, they should stay all right.
What usually happens, though, is that the previous bootloader gets overwritten when you install a new operating system, so in some cases your bootloader configuration might change to the defaults set by the new operating system's installer. Usually this isn't a problem, because most Linux installers will find your other operating systems including Windows and you don't have to add them to the boot list manually, but in some cases you might -- and if you do, don't panic: even if one of the "old" operating systems doesn't show up in the boot list, it doesn't
necessarily*mean it's deleted (but it might if you did the partitioning wrong
).
Lastly, if you at some point want to remove all other operating systems except for Windows, you probably want to get the Windows bootloader back too (instead of Grub, Lilo, ...) Then you need the Windows setup disc (or need to boot on-harddisk repair utility if there is one -- use existing Linux bootloader to do this) to run a "recovery console" (in case of XP or newer) and use it to fix the bootloader (command "fixmbr", sometimes "fixboot" is also needed).
Put short, you can have several operating systems (as long as you don't run out of disk space) side by side. Just pay attention to your choices when partitioning the disk during setup, because that's the step where you can trash your other operating system installations.
