When installing Windows Small Business Server 2011, the product key included with the product may be rejected as an invalid product key.
The cause of this issue is that it is occasionally necessary to generate new blocks of product keys for a Windows SKU. When this occurs the code to validate the product keys is updated and distributed via Windows Update. However, it is impossible to update the product key validation code on install media that has already been distributed. This results in a situation where the install media for SBS 2011 will generate a false product key failure for a key that is indeed valid.
To work around this issue all that is needed is to skip entering the product key during Windows SBS 2011 install. Once the OS is installed follow the steps below to add the product key:
Click Start, right click on Computer and select Properties.
On the System Properties screen scroll to the bottom and click on the link labled "Change product key".
Enter your product key when prompted and continue. SBS 2011 should now be activated.