All Windows XP discs are bootable anyway, and it is very unlikely to be worthwhile installing Windows XP on a machine that isn't capable of booting from a CD.
You can't install XP "into" Windows 2000. You can, however, install XP over it, thereby reformatting the disk and erasing Win2K. You can also set up a dual-boot setup and have both operating systems...
you must reformat the drive XP is on-Then install 2000 as a new OS.
Because it is a downgrade-not reformatting the drive will cause XP to not allow the downgrade.
Open "My Computer" on the Desktop, right click on the disk you want to format under "Hard Disk Drives" and then click on "Format". Select the required options and then click on "Start"...