1 - Go to your DP game directory: right click on the game in your Steam Library - properties - Local Files - Browse local files
2 - Launch the "PhysX_SystemSoftware.msi" file in the "redist" directory and execute the Uninstall process.
3 - Re-launch the "PhysX_SystemSoftware.msi" to install it.
4 - Now launch the "DPLauncher.exe" as Administrator.
5 - Voilà! Have fun! =)

PS. If still doesn't work, simply right click on DP application: properties/compatibility and there, set as Windows XP (pack 2) and check the box to run as admin. This will 100% make it work.