First defragment your hard disk using Windows defrag utility, or Defraggler; or Auslogics Defrag tool (all freeware)
Then check how much free Gb space you have
Then create 1 extra partition on your hard disk using Paragon or Easeus partitioning freeware, with enough Gb to store your data, and not bigger then the above calculated free space
Then move all your data, being non-progam files, to that 2nd partition. Dont forget your browser bookmarks, emails, and calendar stuff, which is usually saved in the c/program/... folders.
Then insert Win 7 install disk and choose some kind of custom install option where you are able to choose the partition you will be installing Win 7 on. Obviously choose partition 1, being the one without the data on!
(Even more safe wold be to also take a backup of your data to an external drive or DVDs, in case somethig goes wrong during install)
brgds,
mack
Edit: now i see that you actually specifically want to overwrite everything, and not prevent some data to be overwritten.... Well; then make sure, at install, to make sure you choose the install option with the word 'format' in it