PopSub.exe is InterMutes Popup subtractor. This is a popup ad blocker. More information can be found here.

Quote:
PopSubtract is InterMute?s intelligent pop-up ad blocking utility. Designed for ease-of-use, superior blocking and super fast download, PopSubtract keeps both Internet browser Pop-Up and Pop-Under windows from appearing. PopSubtract even blocks those annoying Windows Messaging Pop-Ups!