De-Authorising Computers When You Are Not On Them.

I am able to see what computers my music/iphone are registered to

No.

See this-> About iTunes Store authorization and deauthorization
"Make sure you deauthorize your computer before you upgrade your RAM, hard disk or other system components, or reinstall Windows. If you do not deauthorize your computer before you upgrade these components, one computer may use multiple authorizations.

To deauthorize all computers associated with your account
If you find you have reached 5 authorizations, you can reset your authorization count by clicking *Deauthorize All* in the Account Information screen.

1 - Click iTunes Store in the menu on the left side of iTunes.
2 - If you're not signed in to the store, click the Account button, then enter your account name and password.
3 - Click the Account button again (your ID appears on the button), enter your password, and then click View Account.
4 - In the Account Information window, click *Deauthorize All.* "

The way I do it is sign into your account and click VIEW MY ACCOUNT. Then it will show how many computers you are authorized to and it gives you an option to the right enabling you to deauthorize all computers for your account. Then simply reauthorize once the computer(s) you want to be.

Hope this helps, let me know

it gives you an option to the right enabling you to deauthorize all computers for your account.

Anyone reading should note that the option to deauthorize all computers only appears if you have the maximum of five computers authorized. The "deauthorize all" option can also be used only once per year (unless something's changed in that regard that I haven't heard about).