The product key for the OS (XP) is usually on the computer. I'm not
aware they put the product key for Works on the computer. Either in
paper work, or on the CD case, or ... as they evidently told you, on the
computer case.
Sugget you go back to the vendor from whom you purchased this and tell
them that the product key doesn't work (Item 4 on original list).
Ensure you specify which product key you want Works or XP. Don't
confuse them.
Hope this is useful to you. Let us know.
rms