To use the adb command, you need to always navigate to c:\Program Files\Android\android-sdk\platform-tools and run adb logcat or adb devices etc.
But a simple tips, u never need to navigate to the platform-tools folder manually. Simply add the platform tools path as your System Variable (in Environment Variables - u want me to tell you where is it ? :) )
Do that, and Walla, u can issue adb command anywhere in the command prompt and u never need to navigate manually. If you dont have a PATH Variable already in the System variables, Create a New One.
If this trick doesn't work, then add the same path in the User variables as well. U must be having the Path variable in the User variables. (u must have installed JAVA or Python right)
If you are facing any problems, please comment !!!