With due respects to markipad and everyone else who is endeavouring to solve this problem.... we all appear to be going around in circles, re-trying steps that have been tried by others before. Undoubtedly, the solution is probably somewhere within all this endeavour.
However, we may all need to accept the fact that Apple's programming for iTunes 10.4 update is flawed!
And, I repeat as I said in another forum: "Hi Markipad, thanks for the lead but it all appears to be same old, same old. We, the Apple users did not create the problem, so why should we be looking to solve it? It is an Apple product, an Apple f*up and Apple's responsibility to fix it! They need to fix it! If they can't fix it then they may lose 1,000s of users!"