It took me forever and a day to get Homesight 2.6.18 installed. Had to install 2.5.12 from the disc I have then update it to 2.5.14, then 2.6.17 and finally 2.6.18

First, uninstall what you have installed. Download the Motorola Driver Removal Tool and run it. Reboot your computer. Second, open up your registry and search for any "MHS", "Homesight" and delete them. Reboot your computer, then proceed.

Install the 2.5.12 disc first.
After it is installed start it up, add a camera or 2.
Close the program.

Install the next update, start the program, quit it, continue on to the next one.

I didn't have any problems installing it if I did the above.
Make sure you don't have any newer drivers on your system first.

Here are links to the updates:

Motorola Homesight 2.5.12 disc image

Motorola Homesight 2.5.14 update

Motorola Homesight 2.6.17 update

Motorola Homesight 2.6.18 update