On OEM installs (Dell, HP, Acer, etc.), the Product Key that comes attached to the system belongs to only that system and cannot be transferred to another.  OEM versions of Windows are generally included with your PC for a very low price, but part of the price you pay for that discount is that it is tied to that system, vs. a Retail copy of Windows that can be installed on whatever system you want, as long as it is only on one at a time.  
I recently upgraded my Dell Desktop system to Windows 7 and I then installed the previous (unused) operating system  (Vista Business) on my laptop, which I assumed would present no problems as I am still only using it on one computer. However, Windows is telling me that the product key is invalid for activation. I bought an XP installed system with Vista Business as an upgrade option so that hasn't been used or registered so I'm surprised it hasn't been accepted by Windows. Or is it simply that operating systems that come with specific computers aren't transferable? I'd appreciate it if someone could put me out of my misery  as I've been looking for an answer for days, Thanks in advance!      
Can you transfer your Dell operating system to another computer if you upgrade?