i do not agree with what mike said. if you have data on a partition already and try to install an operating system on it it will leave the files alone. if it does require you to format it would give you a warning so do not be afraid to try. it gives you a chance to back out of the process.
i say this because you can install an operating system over an existing operating system and leave all other files intact. i have seen this with recovery CD's in their options menu.
you must meet several conditions to install an operating system on a partition that already has data on it besides the obvious ones like the system requirements (processor speed fast enough, memory is enough, have enough space to install, ect.)
one condition is that an operating system can only be installed on a primary partition. you cannot install it onto a logical drive contained in an extended partition.
the second thing you must do is make it the system & boot partition so it is the first to boot. after you install winXP on it go to the disk management snapin and set the partition to 'set as active'. this is because another partition can be set as a boot partition even if it has no operating system on it like winXP.
this applies to winXP and probably win2k i would think, win98 and millenium are much easier, all you need to do is set active partition and make sure it is a primary partition. it is much easier than i make it sound, but i like to give out detailed info like mike in some situations...
so mikesgroovin i know how you feel bud, (read your networking post)