Just ring up Microsoft and yell at one of those little robots at the other end to fix it, they are use to the abuse of failed activations among other things.
OEM backup is on the harddrive itself, find the \i386 directory on the harddrive or your cdrom and run winnt.exe to install.
On a holographic label with the OEM versions purchased with a piece of hardware will be the key to use.
Activation is usually done over the internet if the computer is connected to the internet, otherwise, it can be accomplished by copying the alphanumeric string from the activation screen and make a phone call to the phone number [Usually Toll Free] supplied on screen.
Any major hardware changes will kill the activation and it will have to be redone.