The location of the .net framework is not in your PATH environment variables by default. You can add the PATH youself, or use the ".net command window" (Start->Programs->Visual Studio->Visual Studio Tools->VS .net command prompt. Something like that, yours will differ). This sets the correct PATHs and opens a command line.
The alternative is to open a command prompt and just CD to change to your .net framework location where the .exe files are stored.