You want CDC. (Communication Device Class)
It looks just like a COM port to the computer but uses USB.

For the 16C's there are some examples in the USB folder inside the PBP folder.

I've never used the old "Low Speed" devices, but I think it would be easier with an 18F2550 or equiv, since they work with High Speed, and there are more examples available.

Have a look at this ...

USB CDC Communications for Dummies! (Squibcakes)
http://www.picbasic.co.uk/forum/showthread.php?t=5806