How does one copy cassette tapes to computer?

On gain, cassette decks follow the old convention of 1/4 to 1/2 volt output.
So they will not be likely to overload the input.
(unlike a CD player or DAC, with 2 volts out)

Thanks very much for the responses, everyone! I will relay this info to my friend, and also possibly make use of it myself when I get the time!

I use a Nakamichi Dragon into an M-audio Delta 1010, running Steinberg Wavelab recording and mastering software. An M-audio Audiophile 194 sound card would be less expensive and almost as good. Unfortunately, many recent brand-name desktop computers lack an extra PCI slot. Aside from the playback cassette deck, the quality of your A/D converters will be the limiting factor. Unless you are a computer wizard, there is a steep learning curve to configure and learn to use this stuff.