As long as you dont format or delete a partition, it will stay there as long as you like. When you do an install, you are presented with a choice ( if you use a graphic installer such as present in Mandrake ) as to whether you want to use the whole disk or existing partitions. By chosing to use existing partitions, you can fiddle with how you want your install to go.
For example, if you had a partition scheme as follows already on your drive:
/dev/hda7 <windows installation>
you could unmount the the /home and /usr partitions and do a format of the /root partition, install another distro ( like Mandrake 9 ) into this partition and then mount /home and /usr on /root if you like without losing the /home and /usr partitions.
Thats the nice things about using partitions like this.
You can play around with mounting and installation options all you like as long as you keep your partitions straight.
Keep in mind that you can also chose which partitions will be modified or added using fdisk at a bash prompt too. You dont have to use a graphical installation program.