Okay, you're going to need several objects for this.
First, to get the files to operate on, you'll need
aqFileSystem.FindFiles. This will return an object that you can then use to iterate through your list of files and operate on them. You'll need to use the path property of each file to get the file itself because you'll then need to use
aqFile.ReadWholeTextFile on each file to get the contents into a string. You'll then use
aqString.Contains to check the contents of each file for your substring. If it returns a -1, you can assume that the string is not found. Anything other than a -1 will mean the string is found.
Does this help?