If you have Windows 7 Enterprise installed already in trial mode you should be able to just activate the copy you have installed. Click Start right click Computer and go to properties. At the bottom you should have an option to Change Product Key. When you click that it should walk you through activating your copy of Windows.
Otherwise here is the link to download Windows 7
Pretty sure Noel is right, at least that's what I was going to say. If you are worried about your files, you could always run the Windows Transfer Wizard, and reapply the files when you did the clean install. The downfall to that is you will have to reinstall all your programs.