There are many reasons you might want to download Windows XP. For example, if you need to install Windows XP but you've lost your original XP Setup CD, downloading Windows XP would get you out of that bind.
There are many places online to download Windows XP but none of them are legal. Windows XP is not distributed online so there is no legitimate way to download Windows XP, even from Microsoft.
No matter where you find it, any XP download you come across online is illegal. The only legal course of action when you need a copy of Windows XP is to actually buy a new copy of Windows XP. It's that simple.
It's important to note as well that even if you did download Windows XP, what you would get is an image of the Windows XP Setup CD.
In many ways, however, what you actually pay for when you legally purchase a copy of Windows XP is the product key . This unique number is required during an installation of Windows XP. So even if you did download Windows XP, you would still need a valid XP product key to install and use Windows.