RDC for the Mac. It supports more features than the Unix based RDC clients including audio, drive redirection, etc.
The computer that your trying to control has to be connected to an internet connection with a static IP address. Then you need to port forward Port 3389 to the IP of the PC your trying to access on your router.
Depending on if it is a computer on a domain or not you will also need to add the username your logging into to the Remote Desktop Users Group. On XP/Visa/7 right click on My Computer/Computer, properties. In XP there is a Remote tab, and Vista/Win7 there is a Remote Desktop link on the right. On both there will be a Users/Groups button. You have to add the username and the user name must have a password.
If its a domain account you need to do:
domain\username
If its a local account just the username but again you must have a password assigned, and if its a domain account then RDP cannot be disallowed via a policy.
Make sure your going through Remote Desktop, not Remote Assistance which is something else.
RDC for the Mac works great, I spend most of my day in it connected to customers servers.