No, Simply formatting the active partition would actually leave you without a working operating system at all
You have to create a new boot system on the Windows 7 partition and
remove the existing one on the XP partition.
Formatting will just remove the XP one without rebuilding the Windows 7 one - thus you would not have any boot files
One thing you could try is EasyBCD - have a look on their site I think there may be a way to build the Windows 7 boot system from within XP, but not sure how complicated this is http://neosmart.net/dl.php?id=1