safetyfast,
OK, I'm with you... 2 situations:
1. Building a new computer yourself: Just put the old hard drive (which is now in the enclosure) in your new machine. Use the drive as the primary and it should boot even with a new motherboard and chip set, etc. Then you're back in business. MS will undoubtedly ask for a you to activate your copy of Windows.
2. Buying a new computer: If it's a purchased machine, you can hook the enclosure up to the new computer and it should show up in Explorer - but don't worry, it will NOT boot its copy of Windows in that situation. Then you can move your files at will from the old hard drive and put them where you want on the new machine...
When you ask "De-authorize the old PC..." I'm not sure what you mean. Do you mean "De-activate your copy of Windows?" If so, NO, you DO NOT need to do that - if you build your computer and use the HDD as primary, you will have to do a new activation anyhow because MS will recognize changes in the hardware, unless it's a replica of the old hardware. If it a new buy, it will probably already be activated. But your old copy of Windows is still good - you can use it on another machine or put it away for future use...
Does that answer your question? If not, ask again...
Noel