I still track in X1 because I thoroughly enjoy editing in layers over lanes.  I'm very quick and it saves me a lot of time and makes my clients happy.
 
I then mix in X3 because I'm trying my best to stay current but simply can't make 'take lanes' work for me.  I get hung up a lot, but that's for another topic covered to death.
 
So I'm installing X1d x64 (currently use x86 version).  I'm unable to install the X1d update because I get to the serial # screen and it continues to tell me my number is invalid.  I have updates up to X1c (x64) installed, so my number is valid and I'm 99.99% sure it's not user-error in an obvious way.
 
Any ideas as to what could be going on?

X1 Producer or X1 Producer Expanded? Producer Expanded has it's own serial number and requires the X1 Producer Expanded product be installed after the X1c Producer update and before the X1d Producer Expanded update. BTW installing the X series out of sequence (i.e. X1 or X2 after X3) can play havoc with the QuadCurve EQ http://forum.cakewalk.com/FindPost/2915734

You may also need to make sure to launch the initial X1d with explicit Run As Administrator for the registration to complete properly.
 
Bob Bone
 

Make sure you are typing in your serial number and not your registration number - can't tell you how many times I made that mistake (and probably will again in the future).

It was indeed the X1d expanded thing.  I totally forgot that was a separate serial!
It installed, but I continue to be told that it wasn't installed correctly and to revert to my initial install CD.  So I've just gone to X1c and will track in that.  Meh.  One day I'll figure out how to quickly edit in X3/track-lanes.

From X1c Producer, install the X1 Producer Expanded from your store account, this will take the X1 Producer Expanded credentials to authenticate. Once running X1 Producer Expanded, apply the X1d Producer Expanded Update. The Expanded features don't do much for tracking so you may want to update to X1d Producer and skip the expanded upgrade.