to do this with any level of quality, you will need a tape deck with Line Out phono sockets and a lead with 3.5mm jack plug on one end and 2 phono plugs at the other. You connect it to the PC Line In socket. Use nero wave editor to ceapture the casettes and away you go-.
incidentally, I'd suggest you keep on to the originals - you may need to do another copy later on if technology moves on and you want them in another future format, or you lose the hard drive... Note that cd-rs do die - i have some from 7 years ago which have deteriorated , and 20 year old tapes which play perfectly.