When it comes to searching a string of text from multiples files at once, Windows Grep is an awesome tool.
It searches a wide range of files including text files, MS Word documents, Html files, Zip files etcetera for the string specified by you.
You can also use it to replace text globally.
A must have for programmers. And it is free. Cheers!
Download Windows Grep
Requires Windows 98, 2000, XP, Vista.