Steam only has a 32-bit version, so it's quite easy to transfer between Windows 7 versions. Here's what I did:
- Copy the
Steam folder from C:\Program Files\ to backup.
- Install Windows, whatever, do your thing
- Install Steam.
- Copy the backed up
Steam folder to C:\Program Files (x86)\, overwriting what you just installed.
Steam's pretty good at detecting anomalies and fixing them or updating, so I wouldn't be too worried about having conflicts after you copy the folder over. I certainly didn't have any problems.
(By "protect" I assumed you meant you want to keep your downloads, so you don't have to download them again. Steam will let you re-download/re-install your games, of course, so you don't need to de-register them or anything. You might want to de-register games that also use a third-party activation like EA's or Games for Windows Live, if you can, but that's a different question.)