If you get good quality sound when recording LPs to your computer, then the soundcard isn't the problem.
Based on that tidbit of info, it sounds like one of two things. Either your problem is either the cassette deck or it could be that you have the deck's output level mismatched with the line level input of your computer. The software I use for transferring old analog sources is Adobe Audition and it includes an input volume display that lets you know when the input volume is adjusted correctly. Does Nero have a similar feature?
Based on that tidbit of info, it sounds like one of two things. Either your problem is either the cassette deck or it could be that you have the deck's output level mismatched with the line level input of your computer. The software I use for transferring old analog sources is Adobe Audition and it includes an input volume display that lets you know when the input volume is adjusted correctly. Does Nero have a similar feature?