Prn files date back to the days of MS-DOS 1.0, and they might have had some use then. It's the byte stream sent to a printer, but then redirected a diskfile. So the exact contents is totally dependent of make, model and 'language' of the printer. I know no printer that comes with software that emulates the firmware in the printer (that makes nice printed pages of that byte stream) to make equally nice screens on a PC.
I think that 99% of the current computer users don't know what to do with such a file (you're certainly not the only one). It's quite simple: you type the ms-dos command copy xxx.prn lpt1: and the file is sent to the printer and printer prints the contents.
I don't know if this works in the command processor of Windows 7 to copy that same file to a USB-printer, a networked printer or a wireless printer, but I'm afraid that won't work. But the printer is connected to a parallel port this should be the way to get the contents on paper.
The two main uses of such a file used to be:
1. You could easily print multiple copies of a file by repeatedly issuing the copy command.
2. You only needed the right printer driver on a PC to print to a file, then copied the file to diskette and could print it on another PC that had that printer, but not the software you used to print the original file.
Nowadays things are totally different. We print to a pdf-printer, which makes a pdf-file which we can view and print in any pdf-reader. All we need to be able to do that is to install such a pdf-printer. A common and good and free one is cutepdf from http://www.cutepdf.com/products/cutepdf/writer.asp
If you install it it will make a new (virtual) printer on your PC and if you choose that printer (in stead of your standard printer) it will ask for a filename for the pdf it makes. That's very easy to use.
Carefully read the instructions about installing both cutepdf itself and the (open source) ghostscript.