Your best bet is installing a second harddisk (buy or borrow).
Then get hold of Drive Image/Acronis Trueimage, and Partition Magic or similar.
Take an image and copy that to the second harddisk, just in case. If you select 'compress' it will be quite small. Both DI and TI only image the 'occupied' space.
Then use PM and resize the harddisk down to e.g. 40GB at the beginning of the disk. Perform the task in PM.
Then create the second partition with PM and perform the task in PM.
Finally use PM to format it, it's a lot faster than Windows format.
You can do all tasks or actions in one go, but it is more secure to do them one at a time.
At the end you still have a working XP in partition C.
Now make a new directory on your C-drive and copy the 3 files: boot.ini, ntdetect.com and ntldr into it. These are in C:\ (root of C-drive).
Install W2K on the new partition. W2K will overwrite the 3 files you just saved.
When done installing W2K, it will probably boot straight into W2K.
No problem, you need to copy those 3 files back, overwriting the ones that were left by W2K.
boot.ini may be read-only, so rightclick it, select Properties and UNtick Read-only, and click OK.
Now use Notepad and modify your boot.ini so that it will look like:
[boot loader]
timeout=15
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Windows XP" /fastdetect /NoExecute=OptIn
multi(0)disk(0)rdisk(0)partition(2)\WINNT="Windows 2000 Pro" /fastdetect
The above defaults to XP. If you rather want W2K, change the default= line to:
default=multi(0)disk(0)rdisk(0)partition(2)\WINNT