You shouldn't have to do that on windows 8.1 only windows 10 has tighter driver security.
To answer your resolution question, no, it's not normal to only have 800x600, that's a default
resolution.
Follow these instructions:
Here's the installation instructions for differentsliauto:
1. Go to
http://www.geforce.com/drivers and download the latest drivers (361.43) for your version of windows.
2. Go here:
http://www.gamefront.com/files/25401790/nvlddmkm36143w or
http://dl.free.fr/pghbDMTTa
3. Go here:
https://github.com/EmberVulpix/DifferentSLIAuto/releases and download the 1.3 version of this package.
4. If you don't already have winrar installed, go here:
http://www.rarlabs.com/download.htm and download the version of winrar for your system that is current and run the installer.
5. Unpack differentsliauto 1.3 to a folder of your choice (c:\diffsli13 for example).
6. Unpack the patched driver into the same folder you unpacked differentsliauto 1.3.
7. Run the official video driver installer you downloaded from nvidias' site and let it install the official drivers.
8. Once the official drivers are installed and the installer has finished, go back to where you unpacked differentsliauto 1.3 and you also have the patched nvlddmkm.sys.
9. Open a administrator level command prompt (hold down left-shift & right-click anywhere in the window (don't have any files high-lighted) and select 'open command window here'.
10. Run the 'Install.cmd' file, it should say 'success!' in green text at the bottom of the window if there were no errors.
11. Reboot your computer.
12. Go to your nvidia control panel and activate sli.