What causes a dialog window as shown in the attached document to pop up when my client accesses his office computer via remote desktop and only when he has to log in on his office computer?

You can modify registry
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Tree\ASUS\i-Setup180917]
"Id"="{4960FFCF-AAED-4C16-AE65-B0BE6C318F7E}"
"Index"=dword:00000002"
NT\CurrentVersion\Schedule\TaskCache\Tree\ASUS\i-Setup182418]
"Id"="{4960FFCF-AAED-4C16-AE65-B0BE6C318F7E}"
"Index"=dword:00000002"
NT\CurrentVersion\Schedule\TaskCache\Tree\ASUS\i-Setup130842]
"Id"="{4960FFCF-AAED-4C16-AE65-B0BE6C318F7E}"
"Index"=dword:00000002"

or

Instead of hacking at the registry, you can delete this app from Task Scheduler:

From the start menu search for "Task Scheduler" and open it
Under the Active Tasks section of Task Scheduler (Local), look for tasks that begin with "i-setup" and are triggered at log on of any user and delete them