You haven't given to much information on what you want to achieve here. Do you want your C drive to remain intact? Is your hard drive in multiple partitions? If so are you saying you don't want to risk the other partitions?
If you mean perform a system recovery (Roll back windows to a previous time, leave data untouched) take a look here:
http://www.sevenforums.com/tutorials/700-system-restore.html
If you mean reinstall windows (back to factory settings, but with data left untouched) have you considered performing a repair install? See here for information
http://www.sevenforums.com/tutorials/3413-repair-install.html
Finally you may mean a complete reinstall of windows 7. When performing an installation you can choose whether to format a partition and use that for the install or install over an existing installation (the old install gets moved into a windows.old folder).
In all above cases only the partition with windows 7 on it will be affected (default is C).
Please let me know if you need any further information.