Okay... it can be done.
First off, the term is "install," not "download." You'll want to keep this in mind to avoid some...erm...legality-related comments....
I'd REALLY recommend putting win2k on your "D:" partition. Two windows installs on the same partition would at the best be extremely troublesome; in fact, I'm pretty sure it woulndn't work at all.
I've never dualbooted multiple windows installs so I'm not sure of the exact process required, but you're going to want to boot from the win2k CD, and format your "D:" and install to it, without touching your "C:"
Then, you need to boot into win2k and configure NTLoader to boot both win2k and chainload the win98 bootloader, and give you a boot menu on startup. You do NOT need to buy a third-party bootloader...
Sorry I couldn't be more specific, keep checking and I'm sure someone else will give you a better answer.