There is no add/remove for WinXP. My earlier comment is how to manually remove it. While in the W7 operating system:
-- I would install EasyBCD,
-- use EasyBCD to make W7 the default OS,
-- copy all my Documents, Pictures, Music, etc. from the WinXP drive to another location, like the W7 drive or another hard disk, and
-- then format the WinXP drive.