One of the CD's is a genuine one that came with The Official Ubuntu Book
The other was a download and I should still have the ISO itself which I can check, although as I said they both work on my old PC.
I learned about burning speeds the hard way although it's not always the answer, when I rebuilt my old PC a slipstreamed XP-SP3 CD that I had used before was rejected.
As my genuine ones are both SP2 I had to use one of them and then install the service pack, fortunately I always keep downloaded installers except for normal MS updates.
I get a File Hashes tab under Properties on all my computers: CRC32, MD2, MD4, MD5, SHA-1, SHA-256, SHA-512. There's 7 more that are hidden.
It also allows me to compare files.
I have a number of both CD and DVD RW discs, anything I am not sure about I put on one of those first.