If you want to open Visual Studio command promt without having VS, Then you can try like this. It might work for you.
Step1: First open command prompt using cmd command
2: Now go to "cd C:\Program Files\Microsoft Visual Studio 9.0\VC" and you can find a batch file with name "vcvarsall.bat". Type the name and press ENTER.
3. And now you can RUN any .net commands like "SN", "GACUTIL" any thing.
Try once and responde if it is working for you