Microsoft Outlook is not set as the default mail client?

While installing CRM Outlook, it gives a strange error:"Microsoft Outlook is not set as the default mail client. Please set Microsoft Outlook as the default mail client from Control Panel\Internet Options\Programs, and then re-run the check."

I'm sure the Outlook is my default mail client, just in case I reset it again. but the error still exist! I'm aware of CRM always query Registry to get information in such cases, so open the default application key: HKLM\SOFTWARE\Clients\Mail, the Default value is Windows Live Mail, so change it to Microsoft Outlook, the error gone!