Seeking advice regarding unusual reinstallation method

I would like perform a clean install but have no where to back up to. I have devised an alternate plan that involves installing Windows on a fresh ~20GB partition and migrating the contents of a ~120GB data (and currently main OS, i.e. drive C:) partition over to the 20GB one. I have never attempted such an unconventional method of reinstalling, therefore I seek advice from the community as to whether it is worth doing and if there are any nasty issues I might encounter.

Here is an outline of my plan

1. On a 140GB hard drive, partitions are as follows:
   
   • 140GB partition C: with 20GB of free space.
   • 8GB restore partition which for all intents and purposes can be ignored from this point on.
   
   
2. 140GB C: is shrunk to 120GB and a new partition F: is created from the 20GB of newly allocated space. The hard drive would then have two partitions, a 120GB C: and a 20GB F:.
3. Install Windows on the 20GB F:, leave 120GB C: untouched.
4. Start migrating everything from 120GB C: to the 20GB F:

I surmise it would be very tedious to migrate 5GB (Windows would only occupy 15GB on the 20GB F: ) at a time between the drives and continually adjust the partition sizes so that they total to 140GB (i.e. once the first 5GB is migrated, C: could be shrunk to 115GB and F: expanded to 25GB. Repeat as needed until F: contains all the data that was originally on C:, then finally remove the old C: and rename F: to C: .)

Opinions?

Basically, I want to reinstall without having to copy over to an external hard drive. Instead of reformatting and installing on the large 140GB partition, I would do a clean install on a tiny 20GB partition. It would be akin to dual-booting. Once I have the 20GB clean installation, I could just start moving the data from the larger partition to the new one. The goal is to leave the data untouched and to allow for easy transfer.

Yes, I kind of figured expanding and shrinking so often would probably take eons. Regarding the Costco suggestion: you read my mind. That is exactly how I planned on getting an external drive. Guess I'll just have to wait until next convenient time to stop by Costco after all.

If you want to try this, then the tool to use is free Partition Wizard bootable CD, which rarely fails in resizing operations.

Immediately after installing 7 to expansion partition, use PW to Modify>Set Inactive C: so that you can delete all of the old OS files thereby making resizing of F: go faster.

You could also leave the data files on C: to access from new Win7 partition using this: User Folders - Change Default Location