There is no grep or ack on Windows. Windows 7 has a good search engine that can be used to search for text inside files. However, it requires indexing to be turned on for this feature. I do not like the indexing happening in the background nor do I like indexed information to be stored before I search for something.
As a good grep replacement, I like Agent Ransack. It can search for a particular text in text files and files of other formats. It builds the index on the fly and does not store it for future use. It includes support for searching inside popular binary file formats like Microsoft Office, Libre Office and PDF.
Tried with: Agent Ransack (Release Build 762 - 2010.03.29.47911)