If you've moved all of the files off of the target partition then boot the installer to Clean Install Windows 7.
As long as you don't touch the data partition but instead select the target partition, Windows 7 will install there and see itself as C when booted, assigning another letter to data.
Likewise when ready to reinstall to the data partition, move the data off and boot installer to install there. Windows 7 will configure a Dual Boot menu and each OS will correctly see itself as C when booted.
Be advised this is not a recommended backup method. It's safer to always have your files backed up externally to another HD, DVD, flash drive, or the cloud such as Skydrive which offers 7gb free storage with each WIndows Live ID also gives you the ability to Sync Any Folder to Your SkyDrive Account | PCWorld.