I can get a tape recorded to my hard drive.
But how do I restore the frequency losses at the top end?

You should get a straight "1 for 1" transfer from your tape to PC.

Make sure that you don't have any equalizer settings applied on your recording software that might be causing the top end frequency loss.

Thank you for quick response.
I record directly to Audition to obtain the sound wave,and I am not aware of the influence of equaliser
settings. But I have read here and there that the process of recording from a cassette tape tends to downgrade the vitality of the original recording, and this is what I was concerned about. For example, Wave Corrector makes a big play about this issue.Personally I don't really much about it.

When you connect the tape deck to your PC does it sound the same as the original through your PC speakers? If so it should record the same.

The only issues with loss of quality are usually related to enhancement software which "cleans" the signal ( i.e. removes hiss, pops, crackles, etc. ). If you're doing a straight capture there should be no loss of quality. If there is I would suggest checking your connections / leads and making sure that your soundcard drivers are up-to-date.

Depends on what it was recorded on in the first place and at what speed, and what you`re playing it back on now. I have an old WH Smith cassette deck that makes any tape sound crap. I also have a Tascam 112 which is the dog`s danglies and makes the worst tapes sound good.

Sounds to me like your replaying a non-Dolby-d cassette but with Dolby active on the cassette deck, or possibly the wrong eq settings [eg metal selected for a non-metal tape].
I'd take all claims by all programs other than Audition about their being able to enhance the results of the transcription process with a vey large pinch of salt. Audition has very sophisticated controls for "cleaning up" your recordings - they do need to applied very sparingly after much trial and error. The very nature of cassette noise artifacts namely; hiss, wow & flutter, over saturation, means that they are impossible to remove without distorting the music itself.

Many thanks to MJB, modo 1, and Johnny Walker for their assistance with my cassette enquiry. I shall now take some time to digest the comments.