Missing files from Itunes Library

Hello People,

So I have been moving my itunes library to back up storage and back to an active location. Yesterday I received a new computer (always PC, this time Windows 7) and several of my music files show up in iTunes with a "!" symbol next to them.

When this happened in the past I tried locating the files in the library. This time the symbol shows up next to approximately 4,000 songs and in some of the cases that I have tried to locate the files, I could not find them in my library files.

Also, my iTunes folder contains al regular folders but "Previous Library" folder has a another set of "Album Artwork", "Download", "iTunes music ", etc......and the same thing happens with the "Previous Library" folder inside this "previous Library" folder.

Anybody can help, PLEASE!!!!!

Thanks