As long as you are still using the same Apple Account you were using back then, try the deauthorize and then re-authorize method provided in an earlier post.
For more information about your pre-2008 files: "Protected" files are really old from when Apple sold music in a lower quality (128k AAC) format which required special, account-authorized software for playback. Up until 2012 you could pay a supplemental charge and upgrade to non-protected, 256k AAC "iTunes Plus" but that is no longer available. Depending upon how many files are involved you can investigate the cost effectiveness of several options.
1) Once you get them working again, burn them to a CD which will then perform as any regular CD would for playing and, uh, other purposes. ;-) If you do decide to rip and encode sound files into a lossless format such as AAC or MP3 there will be slight quality loss upon every encode to a lossless format.
2) Subscribe to Apple Music ( Subscribe to Apple Music - Apple Support ) or Match ( Subscribe to iTunes Match - Apple Support ). If they are still available this this will let you re-download them in an unprotected format. Delete the protected format ones first (after backing up to be safe) then re-download ( Redownload music from the iTunes Store on your iPhone or iPad - Apple Support ).
3) Buy them again. This may not be all that easy since it may just say you have already purchased it and not present you with the option to buy it.
