At least 2 partitions on a same HD, or 2 HDs Just install as usual, select the partition/hd when ask the system partiton
You can use nLite to modify installation media in order to change the names of default destination system folders (WINDOWS, Program files, Documents and Settings). This way, with some fantasy, you can install numerous xp's, s2k3's or s2k's on the same partition without conflicts between them in future. Did it myself a few times with success. But MS officially recommends not to do this.
I am kind of puzzled as to why? I know it isn't important, but why not just make another partition? Just morbid curiosity.
Curiosity I'm curious: it would be great if you would answer 911medic. I am certainly not all knowing but, given the GREAT potential for conflicts encountered by having multiple OSs on the same partition and, given the wisdom (if I may be so bold) and ease of placing the alternative OS on a second partition (OK) or second HDD (better), my brain cannot imagine a scenario where your plan would serve any advantageous end. Furthermore, if you employ image backups (which is a sterling practice) you do not want to store them on the OS HDD, regardless. I'm stumped. Monk