The retail Windows can be installed in to more than one PC (not at the same time). OEM and DSP Windows licenses only goes with the original hardware(PC) and cannot be transferred to a different PC.
These are pretty much the official rules.
However it gets a bit murky pretty quickly in practical terms.
Say you have an HP Pavilion HPE H8-1120 Desktop PC. The power supply and motherboard die 2 months out of warranty. A matching replacement motherboard is $200+ from HP. Instead of going with original feature-dry HP board, you go with an Asus Z77 MicroAtx board, and instead of a crappy HP original power supply, you go with a Seasonic 550W unit. All other components stay the same. (this is a real example from the past week).
Now
officially, this is no longer the 'original' hardware, and getting Windows re-activated would cost $100+ for a new license. Imho, this is abusive and makes no sense. By re-using as much original hardware as possible, it makes it economic to just rebuild and get more years out of it before dumping it in the trash. It's still the same PC, just with a replaced motherboard and power supply.
Microsoft is generous enough to allow this re-activation on the phone-in process provided you describe it clearly. "Computer crashed, we are reloading it" or similar and : Bingo, activated. On how many computers is this copy of Windows installed on? "One!".
This is way
way different than taking an original OEM license, leaving it both on a working original computer, and then also using it to activate a completely different computer.