Possibly simple - possibly not...
Do you have a cassette player which has an audio 'line out'? Probably a pair of phono sockets coloured red/white or sometimes black/white.
If yes, you can connect them directly to a line In socket on the PC (usually a 3.5mm stereo jack socket.
Then record using the Windows recorder (Start - All Programs - Accessories - Entertainment - Sound Recorder, or something like that). You can save the file in various audio formats.
If there are no line output sockets, you could try linking a headphone out to the PC line in, but the sound level will be very low. There's various ways round that (possibly) but see how you get on. HTH.