Plug the stereo outs of your Portastudio into the line in on your sound-card on the computer, start up a recording program on the PC which can record more then a 60 second wave file, (as the Windows sound recorder is limited to 60 seconds only?!), start play on the Portastudio, press record on the software program and stop once its finished.
Now you'll have a big, uncompressed wave file on your PC that can be converted to mp3 via other optional software.
If you have a program like Roxio Easy CD Creator 5 or higher installed for your PC's CD burner drive, you'll have all the software needed to do the deed.
Hey Sumo....
If you don't have a recorder on the comp, you can go to several sites for some freeware or shareware. One of my favorites is : http://downloads-zdnet.com.com/2001-20-0.html?tag=zdfd.header5
Go to the Audio section and start browsing. You can also use the search function to narrow down your search. Tons of stuff out there.
If your in the mood to spend a little money, Cakewalk home is pretty cheap and comes with some basic editing tools as well. I think it runs bout $40 or so.
Good luck.