This has been occurring for many people for many years, including myself. I'm not sure what actually causes it since typically only physical damage to the CDs ought to be responsible for files becoming corrupted, yet I know for sure I didn't particularly mistreat my CDs.
It could be that the original CDs were manufactured quite poorly and degrade/damage faster/easier than is normal (all CDs have a shelf life but it's meant
to be extremely long), or perhaps there's some strange Windows problem at the root of it — but whatever causes it the only solution anyone seems to have found is to install the game to the hard drive and then somehow acquire copies of the corrupted files from elsewhere.
Maybe someone can help you with this, or maybe you have your own means of getting hold of them. If not I'd recommend looking into the re-release as from what I gather similar problems haven't been reported with that, and I myself have had a re-release version for many years that hasn't played up like my original boxed CD version did. You can still get the English version relatively easily
, the German version may be trickier though.