Pre-installed software
If the software is pre-installed, the software lives and dies with the PC and can never be transferred to another PC
Full packaged product
With full packaged product software bought from resellers, you can transfer the software to another PC. As long as it is uninstalled from the first PC and everything is transferred with it (the EULA, the COA, the CD and everything contained in the box). You can transfer the full packaged Product licence outside the organisation, as long as it is transferred complete with discs and documentation and the software is uninstalled from the original PC.
Volume licences
With Microsoft volume licence programmes, you can transfer the software from one PC to another, as long as you have uninstalled it from the first PC and you only use the permitted number of copies of each licensed product you have bought. Operating system licenses acquired through OEM, cannot be transferred, even if upgraded through a volume licence agreement.
For more information, please read this white paper which summarises Microsoft's transferring of licences.
Download the 'Principles of Transferring licences' as a Word document (56.5KB / 8 secs at 56 KBps