Activation incorporates the use of internet and browser settings which, if set improperly, can impede activation. You may need to contact Microsoft by phone and speak to a 'real person' and not the automated 'wench' that usually responds. http://support.microsoft.com/kb/950929
If you open your system drive, you should find 'slui' in the C:\Windows\system32 Folder. It should be 342 KB in size and listed as an application. If it is there and slui.exe 4 will not work on your system, you could first run SFC /Scannow to check for and repair damged system files.http://www.sevenforums.com/tutorials/1538-sfc-scannow-command-system-file-checker.html
Speaking directly with a Microsoft representative might be your only option to reactivate. Every time I change hardware, such as a different motherboard or have to reinstall the OS to a different drive configuration, I need to reactivate. A few times I needed to speak to someone on the phone. The biggest hassle was being on 'hold' for over 55 minutes on three consecutive phone calls before I finally reached a living, breathing human. The worst that can happen is that Microsoft will inform you that your product key is invalid, your software is pirated, and you should contact the vendor for a refund. The Windows 7 installation disks are not 'copy protected' and you can burn all of the copies you want, it is the Product Code/activation key that is protected and encrypted. There are more and more pirated copies of Windows 7 being found everyday. The 'pirates' found a way to bypass the Microsoft encryption and distribute this software as legitimate. Microsoft is just now finding and deactivating these illegitimate copies.