I guess the easiest way to explain this remote desktop allows access to a remote Windows computer. When you are at another computer you can use Remote Desktop to connect to your work computer from home and have access to files and applications, just like you want to do with your friend.
Remote Assistance on the other hand allows you to use your Internet connection to access your friends computer to help him, you can see your friends computer screen and communicate through a chat box, you(as the helper) can remotely control his computer to fix the problem he has.
In Remote Desktop, only one user is active at a given time, while in RA (Remote Assistance), both users can activate the keyboard and mouse.
The request for a Remote Assistance session can be limited to certain duration.
Remote Assistance doesn't require you to configure a local account on the computer to which you are connecting. Remote Desktop, however, requires a local or a domain account and its password to login to the remote Windows XP computer.
Remote Assistance can also be used in "View Mode only", while Remote Desktop cannot.
Remote Desktop is only available in Windows XP Professional. Remote Assistance is available on Windows XP Home Edition, which I use.
When you and your friend are finished using Remote Access make sure that you disable it, if an attacker somehow finds a way in, or if an attack is created to exploit a vulnerability in the Remote Assistance or Remote Desktop services, your computer is just sitting and waiting to be attacked.
Remote Assistance and Remote Desktop can be very useful when you need them. But, most of the time you don't.
