iTunes says my computer isn't authorized, but it is.

I had that problem myself just last week, and this is what Apple told me to do. It worked for me.

Choose Start > Computer.
Choose Organize > Folder and Search Options.

Click the View tab.
In the "Advanced settings" pane under "Hidden files and folders" make sure that the "Show hidden files and folders" option is selected.

Click OK.
Navigate to the following location by either typing it into the address bar, copying and pasting it into the address bar, or clicking through the folder hierarchy listed:
C:\Program Data\Apple Computer\iTunes
Right-click the SC Info folder shown and from the shortcut menu, choose Delete.

Restart the computer.
Open iTunes.
Authorize your computer to play your purchases.

It's a pain and something which needs sorting by someone at Apple. I've downloaded albums from the iTunes store only to find that they have repeatedly disappeared from my ATV and refused to play in iTunes without my computer being authorised!! Then I've had to sync again to get them back on the ATV. Until next time!
Solution? Delete the 'legitimate' downloads from iTunes & download from elsewhere. No problem. Regrettable but true.

The other way to approach this is to de-authorize your computer in iTunes, restart it, and then authorize your computer.

That does solve it......temporarily. It may be just my bad experience, but the problem returns. Something is fundamentally wrong somewhere in the system.

had this problem myself after a full Vista rebuild. Make sure you are logged in as an administrator, otherwise authrization does not work correctly, even though the software states it worked.

no problems when logged in as an administrator. You can log back in again as a standard user afterwards if you need to.