The "original file could not be found" thing happens if the file is no longer where iTunes expects to find it. Possible causes are that you or some third party tool has moved, renamed or deleted the file, or that the drive it lives on has had a change of drive letter. It is also possible that iTunes has changed from expecting the files to be in the pre-iTunes 9 layout to post-iTunes 9 layout,or vice-versa, and so is looking in slightly the wrong place.
(Moving your files by hand from one hard drive to another will have definitely confused iTunes.)
Select a track with an exclamation mark, use Ctrl-I to get info, then cancel when asked to try to locate the track. Look on the summary tab for the location that iTunes thinks the file should be. Now take a look around your hard drive(s). Hopefully you can locate the track in question. If a section of your library has simply been moved, or a drive letter has changed, it should be possible to reverse the actions.
(However in this case I'm assuming the files are where you want them, so you need to fix iTunes.)
Alternatively, as long as you can find a location holding the missing files, then you can try double-clicking a missing track, opting to locate it and accept iTunes' offer to locate your other missing songs based on the same location. If that doesn't work then my FindTracks script should be able to reconnect the tracks to iTunes.
See also Getting iTunes & Windows Media Player to play nicely.
tt2