Here are a few questions to ask yourself:
"What's changed about your Mac since the LAST time you used this disk?"
"Was this disk an authentic XP installation disk?"
You have to be very honest in your answer: if not with us, then with yourself.
If your answer to the second question boils down to "no", you already have the answer to your question, "I don't get what is wrong". If not, then look elsewhere.
It does seem like you are reporting a common error for those who have drives which become inaccessible during the installation of XP, especially if the error you are encountering is a WRITE error.
If it's a read error, either the drive itself has faults, or the CD/DVD drive is being "lost" during installation, or (more probable), the installation media itself has developed faults, such as scratches which are too deep or extensive, or the disk has finger-print smudges, or is dirty or cloudy.
Many times, starting over AFTER cleaning the installation disk carefully, and polishing it with a clean and dry, soft lint-free cloth, will work wonders. Even a very faint finger-print smudge can cause Read errors. Using a spritz or two of a non-static cleaner, such as used for LCD displays, on the recording surface before-hand will not harm the disk or its data, and will pay dividends in the long run.
Donald L McDaniel