For dual boot you need 2 partitions. You have only one. You can either repartition, deleting all present data. Or you can use softwares like partition magic to create another partition without destroying present data. You can also buy a new drive and install XP on it, easy
There are linux programs too for partitioning, but you'll have to reboot using Linux live cd like other fellas have mentioned.
If you repartition current disk you can use softwares like seagate disk manager. But be careful, all data on the disk will be destroyed.
When you have two partitions. Install Windows 2000 first on the first partition C:. Then insert WindowsXP CD when Windows2000 is running, follow instructions. Do not select "Upgrade" but selecte "new installation". Select "I will choose partition during setup". Select "Install all files to had disk". Choose second partition D: during setup later.