You did not reveal much about your old Windows XP license so I'm going to GUESS it's the one from Compaq which is also known as an OEM version. These OEM versions die with the machine or motherboard.
Nothing is wrong here. It's a deal struck years ago and some folk get uppity when we have to discuss this.
If you want to continue using XP you'll have to get a new license.
Still out there, but getting more expensive.
Bob
I had a Compaq NX computer that died recently. I bought a new replacement motherboard and installed it in computer. Now the Windows XP won't start and I can't do anything on computer without it. The phone line for the authorization is an automated line. Can't seem to get help from Microsoft. Please help me.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic