Such an old application may not work with modern Operating Systems.
That's possible but that shouldn't prevent installation on an OS from that era (i.e. XP) nor does it explain the invalid serial number message.
No, but what would explain it is an invalid serial number.
The serial number is not invalid, I'm using the same number I used to install it originally and I still have the original paperwork that contains the serial number so I'm sure it's correct.
This is the full product, not an upgrade. The format of the serial number is:
No chance of that, the serial number contains only numbers, no letters
Hmm. I just did a test install on an old XP install with my valid serial number to see if it did the same for me and it went through without error, so not sure what might be wrong here. Maybe try copying the install files from the CD onto a drive and see if that changes anything.