Hi,
Insert the CD ROM with WinXP and restart the PC making the boot from cd ROM...a simple program appears...now you have to create 2 partitions on the Hard Disk, and install WinXp in one of these new partitions (first the Program ask you to format the partition....ok format in ntfs!).
When finished insert the CD rom of Win 2K and restart the pc booting from CD again: install win 2k in the other partition(same mode).
When you start the pc you can boot from the first or second partition (C: or D: )to start win XP or 2k!!!
Bye
P.S. if your cd(s)(with winxp or 2k) is(are) not bootable(s), you have to make it(they) bootable(s) first (try to search how to do with google)