1. What operating system - XP ,Vista etc ?
2. Do you have a sound card? If so it should have its own software and you can plug the phono from your cassette amplifer etc into the input - microphone - socket of the soundcard.(I have Creative Soundblaster X-Fi). Create a folder in "My Music". Then record via the soundcard software.Not sure if on board sound has a microphone input.
If you have no software there is a wide choice - no doubt others will comment, but I have Roxio Creative suite 10. I think Windows Media Player may do it. Too long to go into here but you need to decide what compression you want on MP3.