Dizzy, hi. As much as I like Remote Desktop for it's speed and efficiency, I know that it is complex and difficult to set up. Go to Start -> Help and Support and search on Remote Desktop and you will find some help there. Besides enabling Remote Desktop sessions, you may also need to tell your firewall to allow such connections and if you have a router, you need to set port forwarding on port 3389. Also, the remote system must have a password on the profile you are logging in on and that account should be locked or logged out. The local system must have the right Terminal Services Client which was upgraded in Vista. So if the local system is not Vista, you need to upgrade the Terminal Services Client.
As for the ports, there are a number of possibilities, but the firewall and any routers or gateways between the two systems can cause trouble. If you can experiment with the two systems on the same LAN, that's a great way to get the settings configured. Also, if you want to remote connect to a system over the Internet, then you should really use a VPN connection. Not only does it provide security that Remote Desktop lacks, but it can to NAT traversal right through most routers and gateways. Also, if you are logging in to a work system from home, your employer should provide you with VPN access because they will want their data protected from snooping.