I have authorized 5 computers, but I don't have those computers anymore, and two of them are the same computer but they have been upgraded to another operating system. Itunes is treating them like new computers. I cannot deauthorize them.

Deauthorize the duplicated computer twice, or use the Deauthorize All command documented inthis article, or use the 'contact iTunes Store support' link at the bottom of it.

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Same computer which had a 1BT hard, but I had BestBuy installed a higher TB (4) hard drive. How Can I unauthorized one. Or what can I do. I have previous Hard Drive and I want to move (copy and paste) the, Music and videos, to the new drive. Can you tell me what to do.

Deauthorize all and reathorize the one or ones you want or need.

From the article referenced above by Niel:

Windows users

If you didn’t deauthorize your computer before you re-installed Windows or upgraded your RAM, hard disk, or other system components, your computer might be using multiple authorizations. Try deauthorizing your computer a few times until it’s no longer authorized, then authorize your computer again, and it will use only one authorization.

With Windows it is the instance of the operating system on a particular drive that gets authorized, not the computer as whole. If you've installed a new system drive that will need authorizing separately. You cannot deauthorize the old system partition unless you can boot into it and run iTunes, however you can deauthorize all, then authorize the systems that you want to use.

See Move your iTunes library to a new computer for thoughts on moving the library.

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