I assume the LP sounds fine on the hi-fi - that the stylus is ok etc.
You may have already tried this, but it's worth checking the leads and plugs/adapters. Check the pins are pushed in firmly in the two plugs that go in the back of the hi-fi (if you use them - one of mine comes out of its plug sometimes, gets left sticking out of the hi-fi socket and has to be reinserted); check the two-pin to headphone jack adapter (if you use that instead - mine was a bit loose and I bought a new one from Maplins); check the plug into the sound card socket on the PC.
I have Audio Cleaning Lab 12 deluxe, and I'm really pleased with it.