Open command prompt and get to the directory you want to search from and type:
findstr /i textyouwanttofindhere *.txt > results.txt
For more information on wildcarding and different ways of searching, at the command prompt type:
findstr /?
There are a lot of variations to the command depending on what and how you want to search, just use the /? option and read what all you can and cannot search for.
Once you have the command line syntax the way you want, just remember to include the > results.txt at the end of the line and that will redirect the results of your search to that file instead of the prompt window.
The results file will contain the filename and the string that matched your search criteria.
Good luck!