Under Windows 7 I have a batch file for checking the status of some repos and outputting the returns to a file (with standard powershell issued git commands).
This w
It looks like your git executable is just not accessible for command line use.
Just add c:\Users\[your_login]\AppData\Local\GitHub\PortableGit_[hash]\bin (or c:\Users\[your_login]\AppData\Local\GitHub\PortableGit_[hash]\cmd) to your Path variable. Replacing [your_login] and [hash] with actual data.
But I believe the location of files will change from version to version, so if you're heavy git user, consider installing msysGit. It will add its executable to the system path automatically (corresponding option available during setup).
Even more, there is the project called mysysGit-UTF8 claiming that they have full UTF-8 support on Windows. I didn't notice the difference, through.