Activation issues, Home edition

You might be out of luck. Since your Windows XP came with a Dell computer, it should have stayed with that Dell computer. Dell restore CDs check the motherboard's BIOS and won't install on non-Dell motherboards. Since you have replaced pretty much everything, you technically have a brand new computer and you are required to purchase another copy of XP. If you had purchased another Dell, you probably would have gotten away with it... though that would have negated the need for an OS anyway.

Remember... OEM copies of Windows are not transferrable to a new computer, they are forever tied to that computer. When the motherboard dies, you can re-activate by phoning MS, but without the product key or the ability to activate over the phone... you're in an almost impossible situation. Are you able to boot to the Windows desktop at least? If so, you can use a copy of Magic Jellybean Keyfinder to get your product key and install XP fresh. You'll still have to activate with MS over the phone... because it's highly unlikely that Dell product key will activate automatically. (Especially considering that key is very likely a volume license key). Make sure you tell MS that your copy of Windows is only installed on one computer and if they ask for any further details (highly unlikely), that your motherboard died and you replaced it.