As @Vince says, for the command 'python' to work from the command line, the python install directory needs to be added to your PATH. This should happen during install, but it seems that your machine doesn't have it. You can see the contents of the PATH by typing
from the command prompt. This will show the directories that DOS searches through to find python.exe. If more than one python directory is in the PATH, the first will be used. You can also see if python is in the path by typing
All entries will be listed in order or an error will be given if not found.
You can change the order of entries in the PATH to default to one version of python. Either press Windows button-Break, or go to Control Panel > System, then select Advanced system settings > Environment Variables. You'll have to reopen the command prompt, as the path is only read when it initializes.
If you have two versions of python installed or the python directory is not in your PATH, you can always run it by entering the full path of python.exe. For example