I have X3D installed on my desktop PC and need to install another copy to my recently purchased laptop.....considering my purchases were upgrades all the way from Sonar 3 Producer, do I have to go all the way back to Sonar 3 Producer or can I install just the latest X3D program? I'm assuming I would have to punch in an older serial number to let Cakewalk know I'm an existing customer? I also want to keep all of my plugs that are on my desktop........thanks!

Install X3 and use the serial number & activation code in your user area on the site to activate it.
Don't forget to install the latest update.

Thanks!

Yeah, install X3 then install the upgrade to x3D
 
If you want to use the likes of R-Mix or v-Vocal from X2 then you need to do an advanced install of X2, deselecting everything that you DON'T want installed.
I would do this AFTER you've got X3D up & running

Thanks Jonesey!

Just a note: 
You should only have to download the software once and then save it as they recommend. 
I save mine to my external drive and then copy it over to my 2 computers to run the install. 
In the folder I keep both the serial number and the activation code they supplied. That way I can install it on my DAW which is not on the internet. 
The patches are just patches and are not the whole program, you only need install the latest in the series. 
The thing I love about Sonar is the no hassle install once you have your numbers and software. I just had to wait 3 days for Steinberg to reply to my request for re activation of Wave Lab to move it to a new machine.