Do you have your Windows 7 programs on the Programs partition?
If so, then you'll need to save it if you don't want to reinstall your programs.
If not, you can use free Partition Wizard bootable CD
to rightclick>Delete both XP and Programs partitions, then rightclick Windows 7 partition to Modify>Set Active, and rightclick>Resize to drag left border over the deleted space.
Then boot into the Windows 7 DVD, select Repair My Computer on second screen, click through to Recovery Tools to run Startup Repair up to 3 separate times with reboots to rewrite MBR to Windows 7.
If you need to keep the Programs partition, then in order to reclaim the lower address for Windows 7 you would need to shrink and copy it there, then resize both partitions as you wish before running Startup Repair to write MBR to Windows 7.
Back up your files and a Windows 7 Backup image as resizing operations can fail.