Since you apparently have both XP and 7 on the same machine, before you reformat the XP partition, you might want to check to see if the Window 7 boot loader files are installed there.
This is generally what happens when you install Seven after you already have another Windows OS installed on the same machine.
If you reformat the XP partition and the Seven boot loader files are there, you will not be able to boot into Seven after that. Suggest you use the option in Seven to Create A System Repair Disk before you reformat the XP partition. That will allow you to boot from that disk and run Startup Repair to reinstall your boot loader files.