Get this:
http://audacity.sourceforge.net/download/windows
Along with the LAME plug in.
Get one of these (Assuming you have a seperate tape deck, not a HiFi):
(If you only have a HIFI with a headphone socket as output you will need different leads, one with one smaller plug each end - like above but with the single shaped one each end).
Putting the two 'castle looking' leads into the OUTPUT ports at the back of your tape deck, (or, if you must use the headphone jack with the other lead, put one end in the headphone port) Plug the other into this on your computer:
Where it says 'Line In' (or has an arrow in a circle pointing down into the circle). Set up your windows recording mixer/sounds etc to record sound on the 'Line In' of the soundcard in your PC. To do this, know the name of your soundcard, and find the relevent button in the Control Panel of windows. Try and not use the Microphone option, cos its potentially crappy.
Record in Audacity, play your tape into the computer, with levels in Audacity that just sit below the "red" on the dancing bars at the top right of the screen.
"Save as" MP3 when done. (or "Export to MP3")
Jobs a good one.
Sirius.