If you're talking about RDP (Remote Desktop) access with full administrator privileges, all you need to do is add the user to the local administrators group. This can be done by running the following from a command prompt.
net localgroup administrators <username> /add
The username will need to be fully qualified (
<domain>\<username>) if it's a domain account. Also, the command prompt will need to be run on the server in question by an existing user with local administrator privileges.
Please note that local administrator privileges allows the user to do pretty much anything to the server.