I got new computer and when I try to install studio instruments I am told that I am not administrator....but I am...when I give serial number it says it is already registered but I cant input registration number...not admin....now what>????

I think you will have to run Cakewalk as an Administrator. 
 
Even though you are the Admin, Windows won't let you run in Admin mode unless you tell it to. This is for your own protection, (that's what they say anyway!).
 
Right click the Cakewalk icon and select 'Run As Administrator' and then register SI. That should do it for you. 

Right click on the StudioInstruments.exe file and select run as Administrator