Here is a little bit background:
I was having this same issue and I resolved it in the following way...
I have the NVIDIA "Tegra Android Development Pack" installed and it seems to also have a version of mysysgit.exe with it. TortoiseGit automatically found that installation location (instead of the standard git installation) and auto-populated it in the settings menu.
To correct this, go to: "Settings -> General" and there is a field for the path to mysysgit.exe. Make sure this is pointing to the correct installation.