That sounds like a really silly thing to do, especially since you could have just used Software Update on your computer to get the latest version of OS X.
Pigoo, will installing the previous version bring all the files back?
If I understand correctly what the original poster did...it sounds like the original poster tried to "upgrade" their MacBook's OS from Tiger (10.4.11) to OS 10.5.7 using a friend's MacBook Pro installation disk.
Then after the install...their computer wouldn't boot.
The problem is, the installation disk from the friend's MacBook Pro is probably a "model specific" disk...and should not have been used on a MacBook.
The suggestion of using software update would not have worked...because you cannot update from OS 10.4.11 to OS 10.5.7 via software update...you have to purchase Leopard (Mac OS 10.5).
Regarding my earlier suggestion to reinstall the MacBook's original OS. When you do an install of the OS...it normally DOES NOT overwrite the user's personal files (application's, documents, photos, music, etc.).
The install program looks for enough FREE SPACE to install into...then you get a choice of a "fresh" install where the old OS System folder is removed and a new OS installed, or an "archive" install...where the old OS System Folder is "archived"..and then a new System folder is installed.
The bottom line is...the original poster's files are probably still intact. The original poster simply incorrectly tried installing a newer version of the Mac OS from a DVD that was not made for their model computer, and thus their computer is not able to boot.
A fresh install of the Mac OS from a "universal" install disk...or the installation disk that came with their computer would be the first step to try, and hopefully would correct the problem.
- Nick