For example, is there a way that you can run your web browser or another program from inside a fullscreen program? I'm using Windows 7.
I am not sure if your question is related to being able to run a window within a full screen window, launch another program while you have a full screen window open, or have multiple applications running simultaneously. So, I will answer all three.
To be able to run a program in the same window as the full screen window you are using would require that the full screen application be programmed to support this feature. This is not something you can simply incorporate if the program is truly 'full screen' which changes the Windows resolution. It is not possible to have a program on top of another program when using full screen because true full screen applications take focus. This is different from maximized, though. Being maximized and full screen are different.
To lauch another program while you have a full screen window open can be done by press
You could have multiple applications running simultaneously and simply switch between them using
Many applications will allow you to use