My basic rule of thumb is that if I get emailed twice about an issue, I blog about it. :-) So here is the blog post. :-)

The basic scenario is this. The customer or partner downloads Microsoft CRM 90 Day Trail Keys from here. And then they select the Enterprise Keys, because it is a trial.  The part that they forget to read is here:

Important: Please be aware that you can apply a license key only for a similar or a more advanced version of Microsoft Dynamics CRM 4.0. You cannot downgrade a full or trial version of the product. For example, if you apply the Microsoft Dynamics CRM 4.0 Professional trial license key, you can later upgrade to either the full Professional or the full Enterprise version by applying the appropriate license key. However, you cannot downgrade the installation by applying a Microsoft Dynamics CRM 4.0 Workgroup Server license key. Similarly, if you apply the Microsoft Dynamics CRM 4.0 Enterprise trial license key, you can later upgrade to only the full Enterprise version.

And then when the go to apply a Professional Key to an Enterprise Trail version, they get this error message.

Invalid License Key

There is no magic potion to fix this.  But our friends in support have created a great KB article on how to fix it easily. :-) http://support.microsoft.com/kb/947272/en-us

While I realize everybody may want to just drop the key in and run, this solution beats redoing everything from scratch. And you won't lose any data from using this KB article.

An even better solution would be installing just the Professional Edition unless you MUST use Enterprise. :)