The subject behavior is the result of checking the Always run install program as <ComputerName\UserName> box.

To restore the dialog:

1. Copy / Paste the following to a WarnOnNonAdminInstall.reg file:

REGEDIT4

\[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\DontShowMeThisDialogAgain\]
"WarnOnNonAdminInstall"="Yes"

2. Merge the WarnOnNonAdminInstall.reg file with the users registry, or run regedit /s WarnOnNonAdminInstall.reg.