One of the things I HATE about Windows is that you just can't have a /bin folder to use things like PHP from the command line where ever you are...or so you'd think. It's actually very simple to mimic this seemingly *nix only behavior.
WARNING: CHANGING ENVIRONMENT VARIABLES CAN CAUSE HAVOK WITH YOUR COMPUTER IF YOU DON'T KNOW WHAT YOU ARE DOING! YOU HAVE BEEN WARNED.
Just head on over to Control Panel -> System (or the shortcut, Windows Key+Pause/Break) -> Advanced -> Environment variables -> Choose the "Path" variable -> Add the path to your install of PHP and save the changes. You may have to restart.
When you're all done, open the command prompt and type in "php -v". If you've done everything right, it should print out PHP's version information.