I found the case from your support call, and taking a look at the screenshots it appears that the "Change the size of all items" has been increased, which would currently cause the mouse offset and sharing issues seen at the 1600x900 resolution.
This resolution itself can be displayed through GoToMeeting if the size is set to 100%/default with these instructions (for Windows 8.1):
1. Right click the desktop and select 'Screen Resolution'
2. Click 'Make text and other items larger or smaller'
3. Move the slider bar to 'Smaller' (or 100%, depending)
4. Click Apply
More instructions available here for Windows 8:
http://windows.microsoft.com/en-us/wi...