System backups generally do not save your installed applications; you can do that by making an image of your OS, but since you are installing a different OS, an image will be useless.
You can backup any important files on your D drive and just format C with Windows on it, but all applications and drivers will need to be re-installed. 32bit drivers will not work on 64bit Windows anyway.
It's a major effort but the results are generally worth it.
What about my third question? will the apps installed in d drive work after i format my c drive?
ALSO, DO I HAVE TO REINSTALL BIOS AFTER I FORMAT C DRIVE?