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I would first recommend that you try updating iTunes.
Using Apple Software Update
To use Software Update for Windows, click the Start menu and select Apple Software Update. Available updates will be shown. Click Install to download and install them.
From: Using Software Update for Windows to keep your Apple software up-to-date
If that does not find any updates, next I would move the iTunes Library.itl file to the desktop, then open iTunes and let it scan for media.
You can use this article to locate the file:
iTunes: How to re-create your iTunes library and playlists
1. Quit iTunes.
2. Locate your iTunes folder (this is the folder that contains the iTunes Library and iTunes Library.xml files).
Operating System Default location of iTunes Folder Mac OS X
Microsoft Windows XP
\Documents and Settings\[your username]\My Documents\My Music\
Microsoft Windows Vista
Microsoft Windows 7
\Users\[your username]\My Music\
3. Open your iTunes folder.
4. Drag the following file from your iTunes folder to the Trash:
Mac OS X "iTunes Library" or "iTunes Library.itl" Microsoft Windows "iTunes Library.itl"
That should get your iTunes library back and usable.