OK, here's the deal.
The standard way (as seen on ExpertsExchange.com) will not work on a USB printer.
However, I was able to get it to work with a bit of ingenuity!
To test it, I hooked up my old HP DeskJet 500 printer to my PC via USB. I have a cable that has the standard printer/centronics connection on one end and a male USB plug on the other. I've more or less made my parallel printer into a USB printer. Let's hope this will work for you.
1) Share the printer. Give it an easy, short name with no spaces.
2) Map your new shared printer to an available LPT port. Open a command prompt and type the following:
net use \\computername\printer_share_name\ lpt2
Where "computer name" = your computer name and "printer_share_name" = the share name you just gave it. I used LPT2 since I already have a printer on LPT1.
3) It should then say, "The command completed successfully." To verify this, type in "net use" and it should give you a little table of your mapped printer.
4) If all goes well, go to the directory the file is located in and type in the following:
copy /b test.prn lpt2
That worked for me. Lets hope it works for you! If it doesn't, I'm stumped.