The Installer folder is hidden by default.

Windows Explorer > Tools > Folder Options. On the view tab put a check in "Show hidden files and folders" and uncheck "Hide protected operating system files (recommended)". Accept the warning message and then click Apply.

Navigate to C:\WINDOWS\Installer

Also, go to Add or Remove Programs in control panel and if messenger 7.5 is listed try uninstalling it first, then reinstall.