It's because your MBR isn't clean. go ahead and load your xp disk in recovery mode, and use the 'fixmbr' command at the prompt. this will reformat the master boot record, and with a clean install of xp or win2k, you should be fine.
also, it 'might' be the case that your win2k disk comes with a program called 'fdisk' on it. if it does, you should be able to do the same with it, and from the command prompt, type:
"fdisk /mbr" minus the quotes.
This will do the same that 'fixmbr' does. just rewrites the master boot record.
what happened here, is that windows operates on its own bootloader, not a system-wide bootloader. linux does the latter. (grub or lilo traditionally) removing linux smashed your boot partition, but the mbr is still written to direct to that partition. wiping it forces the system to default to the first bootable partition, which will be windows.