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I am not 100% certain about this, as I’ve never done it, but I do know that you can downgrade from Enterprise to Professional. I think that you have to use a third-party application to do it, and I’ve never used it, so I’m not specifically endorsing anything. I found a guide to doing it for you, but the plot twist here is that your key is expired. I’m not certain how that will impact things. Perhaps you could back-up your important files, say to hell with it, and throw caution to the wind. That’s what I would do, but then again I’ve been to the ER more than most people, so my approach to life does have its drawbacks.
Anyway, here is the relevant link: http://answers.microsoft.com/en-us/windows/forum/windows_7-windows_install/downgrade-win-7-ultimate-to-win-7-professional/cfebbc77-683a-4d32-bf3a-e897619a82f7?msgId=d6190193-c419-4480-858d-8d4a16cfc3ec