Can someone using Windows Remote Assistance share control of my computer without my permission?

No. You must give someone permission to share control. If you have an administrator account, you can prevent non-administrators from giving permission. To do so, on the computer that you want someone to connect to, follow these steps:
  1. From the Windows Vista default Start menu, right-click Computer, and then click Properties.

  2. Click Remote Settings. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.

  3. Click to select the Allow Remote Assistance connections to this computer check box.

  4. Click Advanced, and then click to clear the Allow this computer to be controlled remotely check box.

  5. You can also clear the Allow Remote Assistance connections to this computer check box.

Note: This information provided by Microsoft.