xp key to install on another drive.
If you have Windows OEM version, it is for that machine only. If you scrap the PC you can't build a new one and install it again using the same key.
Note that the XP OS disks are the same, it's the key you pay for.
Let's say you have a hard drive fail, of course you lose Windows and everything on the HD. You can buy a new HD and install windows again, but when you try to activate the OS with MS, it may fail. (because the part nbr on HD is different). Then you can phone MS and explain what you have done, and they may accept what you say.
You can't keep installing windows using the same key, and you will also find the updates and security patches won't work.
If you buy Windows in a retail boxed version, it's more expensive, but you don't have these problems later on, because the key is not limited to one PC forever.