Make sure you backup your Data first.
You may need to copy boot files from XP drive to your Windows 7 drive, before deleting the XP drive.
1) Make sure you have these files on the Windows 7 partition.
2) Mark the Windows 7 partition ACTIVE, Partition - Mark as Active
3) Boot to DVD,& open a Command Prompt,
Boot DVD, & Press Shift+F10 (for command prompt)
Boot DVD, & Command Prompt at Startup
(for command prompt)
4) Delete Win XP partition, Partition or Volume - Delete
5) Than do a startup repair. Startup Repair
Note: You may need to do startup repair 3 to 4 times.
Your OS should be on Disk 0, but it is not.