It sounds like that program wiped out the associations in the registry.
I would suggest taking it to a computer repair shop and have them fix it. I think that would be the cheapest and easiest thing to do.
Other than that, I think that you are going to have to buy Windows and install it (you will have to start the computer with the Windows disc in the disc drive). I would suggest trying to install Windows to the directory where Windows is first. This would simply update Windows and leave everything else in place. If that does not solve the problem, you will have to install it to a new directory which will give you a new Windows with nothing installed. You will then have to reinstall all of your programs.