you want to play an old tape on your beocenter 7700 and record a digital file on your computer?
what type of outputs does the 7700 have?
does your soundcard/integrated sound have an audio-input port?
i know that there are adapters available to get from two wires to a single 3.5 jack however, before you even think about connecting everything up make sure that you aren't going to overload the audio-in. i'm not sure what the max v/a/w rating is on those ports. i'm also not the most knowledgeable when it comes to audio so i'm not sure how exactly that 2x40w compares with different volume levels. perhaps the 40w is max (but i didnt see anything stating max on the site i was on)
once you get an audio-input into the pc then you should be able to record that signal either directly or as it plays on your pc. i don't record audio so i can't recommend any fancy programs but windows sound recorder should work.. at least it did for me when i used my audio-in.
i'm sure one of the regular audio posters will add in their thoughts. might be good to hear their thoughts before proceeding.
i'm just letting you know how i went about it (though i did it from a different type of source)