If you're adding music to iTunes, and it is initially present within the library and can be played/synced with your iPod, but subsequently generates "... the original file could not be found ..." error, the most likely cause is that something else on your PC is conflicting with iTunes and is moving files such that they cannot be found by iTunes. Although there are other cases, Windows Media Player is the most likely culprit - see turingtest2's user tip on Getting iTunes & Windows Media Player to play nicely for steps that will resolve this issue.
The other possibility is that you are adding mp3 files where you don't have the second of these options checked in Edit > Preferences > Advanced:
If you subsequently move or delete the mp3 files, or have added them from removable storage such as a USB thumb drive, then the links held within the iTunes database - which will be to the original location of the file - will no longer be valid. If you do check the second option above, then iTunes will copy the mp3 files that you add into its iTunes Media folder structure (which in general the preferred/recommended approach), so you can move or delete the original files, or disconnect removable storage, without disrupting any of iTunes' links.