I'm doing a simple project where I read an exe file and if I find a virus that I already know, I erase it from the file. I have to test it with a virus (I'm looking for something harmless) that insert itself into a file but I haven't found anything. Right now i am using an already infected file but i do not have the file that infects other files. The virus I do not already know just pass thru :S but i was asked to detect a virus that i do already know and that does something you can see (like a message).
Metasploit has ways to embed executables in other files. You can use it to launch calc.exe, for your example.
The full detailed manual is here, but it might take a while for you to get the hang of it.