Since version 3.12, we provide OTB Applications packages through OSGeo4W for Windows XP/Seven users:
Follow the instructions in the installer and select the packages you want to add. The installer will proceed with the installation of selected packages and all their dependencies. For the otb-bin packages, it will be available directly in the OSGeo4W shell, for example run
For the otb-python packages, you can simply check from an OSGeo4W shell the list of available applications: