Hey Janin,
Speaking as someone who's had the same problem (I'm rewriting my book as well...the first version was REALLY bad) here's what I do. This won't really offer you any help NOW in finding the file, but instead offer you help in the future

It could be pretty hard to find the file now, but here's some ideas: Use Windows search, or open each document individually and use Word search (Ctrl+F) to search for a specific key-word you know is in the Introduction file. That's quicker than reading them all, at least.

Anyways, that could help you find the file now, here's some stuff that keeps me sorted long-term

:
Name things UniquelyWhen I first wrote my story I had tons of docs floating around, some with chapters, some with only a paragraph, some that weren't meant to be in the book but were only notes. And I didn't name them well. I had "Chapter 1 (1).doc" "Chapter 1 (2).doc" and so on. And they weren't even in chronological order, just tons of different revisions of the same writing floating around.
So name things in a way so that you can find them again. Personally, I name all of my chapters like this: "Zaciré Book Rev 2 - Chapter One (Revision 1).doc" It's long, but that way I can know exactly in what revision this chapter goes (1 or 2), what chapter it is, and what revision it is of that chapter (sometimes I revise chapters up to six times for my first draft!). Anyways, that helps me find what I need. And if I'm writing notes about the book I want to find later, I title them like this: "Zaciré Notes - Thoughts about the Enclave.doc" so that I can know that they are specifically notes, and what they are about. And so on for history, characters, etc.
Use SubfoldersAnother thing that can help is specific sub-folders. I have sub-folders for all my character docs, my history docs, my maps of Enderion, and my language docs. And then in the main folder I keep all the writing like chapters and notes and stuff.
Keep Rewrites SeparateOne last thing, that I cannot stress enough is this: keep rewrites separate from the original. I made this mistake when I started rewriting my book. I just dumped all my newly rewritten stuff into my old folder...it took me about a month to sort through and compress all of the old stuff into one word document that I could then safely move away from the new stuff.
StoryBookAnyways, one last thing. If you're the sort that likes having all your writing sorted and stuff, there's a great program called Storybook for windows. I use it mainly for sorting my character development and stuff but you can also use it to sort your actual scenes, chapters, and writing.

eruheran