Hacker News Reader

If you want a GUI tool, I like GrepWin[0] - you can have it install an Explorer context menu item, right click, hit GrepWin, then you can search. Supports simple text searches as well as regex. You can filter by filesize too. Been using this for years, really useful tool.

On the command line, I just use grep (a cygwin version of it, installed with Git for Windows, I think).

[0] https://tools.stefankueng.com/grepWin.html

If you use TotalCommander it's just selecting the folders and Alt+F7 for advanced find/grep combination

Notepad++ and kate text editors can do that

Check out Binary Fortress' FileSeek