"findstr" is not recognized as an internal or external comma

Hello
I'm trying to install some mega-mods(like-TDD,TS) and connect them with Big World Fixpack.Mega-mods install almost propelly(what I mean is-NPC,quests,shops,show up but I cannot go to any location-game crashing and I'm always landing in windows) but when I try install big World Fixpack in Dos-windows shows up:"findstr" is not recognized as an internal or external command,operable program or batch file."And installation is aborted.
Sorry for my english
may someone help?