How do I put all my songs back onto iTunes?

Okay I'm going to try to make this question as simple as possible. =) I just installed Windows 7 Home Premium onto my desktop last night and backed up all my necessary stuff onto my external hard drive. I keep all of my MP3's in one folder on my external hard drive. When I got my iPhone about a year ago, I decided to only put CERTAIN music into my iTunes library and simply sync it all onto my iPhone so instead of having my entire huge MP3 folder (which wouldn't fit on my iPhone anyway) on there, I could pick and choose which songs I'd like to have on it. In addition to a few hundred songs hand picked from my MP3 folder, I also added 20-30 CD's worth of music onto my iTunes library to load onto my iPhone. Halfway into sync'ing it all onto my iPhone, I decided to uncheck the box that says "let iTunes manage your music". Before when I had that box checked, iTunes converted the MP3's I added into my iTunes library into M4A files and put them into a separate folder on my computer. So when I was all ready to reinstall iTunes on my computer with my new Windows operating system, I found my iTunes music folder which i THOUGHT had all 800 songs that I have on my iPhone in it, so when I just add it to my iTunes library, all 800 pop up, and bingo, there it all is and I'm all set. WRONG. Only 550 of the 800 songs appeared in my iTunes library. So what do I do? Can I upload it from somewhere or do I REALLY have to re-do the ENTIRE library from the ground up, ripping CD's and hand picking MP3's from my MP3 folder? That would take FOREVER. Please help. Thanks. =)