You have to use a high-speed cassette copier (these copy 90 min cassette in 5 min). Connect the audio out to your PC and record the audio in high-speed and save it to a file.
Then use winamp pace maker plugin and winamp diskwriter to slowdown the song in about 5 seconds. This gets normal speed and about the same sound quality as the original cassette. Conversion to MP3 will take a bit longer.
If you don't own a high-speed copier then you can get one for under USD 100, e.g. http://minneapolis.craigslist.org/ele/972677155.html