Boot into Windows 7 and type msconfig in the start-search box.Click on the boot tab and check if the windows xp is listed there.Open disk management,right click on XP partition and select delete volume.Make sure you back up all your data from the XP partition first.
Now open msconfig again,click on boot tab and delete the Windows XP from the list.Restart your system.If windows 7 doesn't boot up then try Start up repair
Startup Repair to repair the MBR boot files.