Hi, It's really easy i record my electric guitar through my pc all the time. To record from a line in open the audio propeties by doule clicking the speaker in the system tray go to options -properties and click the recording option, make sure line in is selected in the check list and click ok. Make sure line in has a check in the select box and then set the recording volume, its usually best not to set this up to full whack else you get a bit of distortion. Then open up a bog standard recording utility such as the sound recorder that comes with windows hit record and press play on your hi-fi.
If all went well you should now have a .wav file of your cassette. O.K I've forgoten to metion something here (sorry) sound recorder only records a maximum of 60 secs at a time, so you might need a different recording utlity.
I'm not familiar with many software packages that do the vinyl or cassette to cd conversion but one you might want to look at is steinbergs clean (website below), this is usefull because it filters out hum and fuzz from your old recordings.