You can use the Cisco Secure Access Control Server (ACS) for this. This is RADIUS/TACACS+ software that you can install on various versions of Windows Server 200x.
You can perform Authentication, Authorization and most import for you; Accounting. The server keeps track of who logged in, when he/she did that, how much traffic passed by, how long he/she stayed connected, etc etc.
More information on the Cisco Secure ACS can be found here: http://www.cisco.com/go/acs
Please rate if the post helps!
Regards,
Michael