Hi BookWorm,
your question is about one of the few topics where more than 1 way is "correct". You will see that the mileage varies and there is no single correct way of doing it. I will describe my way of doing it, then make room for others to explain how they do it. Please note that it's more than a year ago when I last installed a Win OS. (That was XP Pro in a dual-boot environment though)
First: I don't like the boot manager that comes with OS/2 or eCS. This is a long term hate relationship. I found that among all the possible boot managers, the OS/2 thing would be the last one I would use. My preferred one is Air-Boot since it is easy to configure, stable (even when Windoze has corrupted the boot sectors

), requires no disk space and is extremely flexible in defining which OS has access or visibility of the other OS's partitions.
(Air-Boot is freeware, available at
http://hobbes.nmsu.edu/h-search.php?sh=1&button=Search&key=air-bootv105&stype=all&sort=type_name&dir=%2F)
Here's how I do it:
(Note: This is not suitable for you since it starts with installing XP on a "blank" HDD... anyway...)
1) Install XP and make it create a partition of the desired size, reboot until ready-to-use from HDD
2) Install eCS, make it create a new bootable+installable partition of the desired size, install
3) Install Air-Boot and configure both OS's partition visibility so they only "see" their own stuff
This worked for me for years and I'm very happy with it. It would even allow me to install Linux in addition (or whatever OS) and be able to specifiy which of these OS can access which other partitions. I could even create a "shared" partition which is non-bootable and all OS's have access to (as long as they have a filesystem driver that recognizes the partition).
I chose this approach because:
The most stupid/egoistic OS is the first one to be installed (step 1). By "egositic" I mean that XP is not "aware" that there are other operating systems under the sun - as we see by the way it uses the hard disk during install. "Stupid" means that it doesn't recognize other partitions and file formats which will most likely make it fail when there is something else than what Microsoft expects a hard disk to contain during install (anything else than Microsoft stuff that is). So let's just make XP believe everything is fine ...by installing the other OSs later

When eCS is installed (step 2), it has activated its own bootable partition, making XP basically "vanish". It's still there but can't be booted. Now we need a boot manager and this is where step 3 happens. Of course, the whole shebang is feasible with the eCS|OS/2 bootmanager as well... but like I said above, AirBoot has so much advantages over it that I personally will never ever again use the OS/2 BM.
Hope this helps.
Now here come the others' suggestions... and I guess Ivan will be on of the first to comment on how to solve your problem the smart way and in no time.
Cheers,
Thomas