Remote desktop allows you to have access to your computer even when you are not physically present new it. It gives you the chance to operate a computer from another computer. As for remote assistance, it is usually used when you want to give someone accessories to your computer, as you, as the administrator, needs to give permission to the other party.
You have the remote desktop service in your Windows XP professional. Meanwhile, you can get remote assistance in your Windows XP home and edition and Windows XP professional edition. If you want to access the remote desktop and remote assistance through your Windows OS, you will have to go through by pressing or clicking the start button.
Remote desktop is under "Accessories" under "All programs"; while the remote assistance is accessed by inviting a friend through the "help and support" button. Lastly, remote assistance requires the owner of the system, while remote desktop does not necessarily do.