Adobearm.exe will be found in your c:\Program Files\Common Files\Adobe\ARM\1.0\ folder (version number may vary).
Adobe ARM stands for "Adobe Reader and Acrobat Manager". It installs with recent version of the popular Adobe Acrobat PDF reader.
Like Reader_sl.exe, this file will run on system startup unless you use WinPatrol to disable it in your Startup Programs.
You'll find more information and the latest version available for
download at http://get.adobe.com/reader/.