Your data will only be removed if you do an Erase and Install. A default install of Snow Leopard from the DVD will always leave your third party apps, files and settings alone. Only the system files are replaced with new copies.
If at all possible, you should still make a backup of the drive first in case something goes wrong reinstalling the OS.
If I couldn't recover and was forced to install a fresh OS, is it possible to recover data after having the fresh installation?
Not much, but you can install OS X on a extenral drive, boot from it and recover files, then do the erase and install.