If Windows 7 was installed last it likely wrote its boot files to the XP partition which is signified by XP wearing the System Active labels in Disk Mgmt. So if you want to remove Windows 7 you can just delete it.
If you want to remove XP the normal procedure is to mark Windows 7 partition Active then run Startup Repair - Run up to 3 Separate Times
until 7 wears the System Active flags, then you can delete XP.
But because there can be complicating factors I would first post back a screenshot of your maximized Disk Management drive map and listings:
1. Type Disk Management in Start
2. Open Disk Mgmt. window and maximize it.
3. Type Snipping Tool in Start Search box.
4. Open Snipping Tool, choose Rectangular Snip, click New, draw a box around full drive map and all listings.
5, Save Snip, attach using paper clip in Reply Box.
Or from XP make your Screenshot with Paint
Tell us what is on each partition and exactly what you'd like to do.