I think both answers given are wrong.
There are two main packaging types for Windows: OEM and Retail.
OEM is for installing on new PCs and once OEM Windows is installed on a PC it cant ever (legally) be moved to another PC. If the PC "dies" the OEM copy of Windows on it dies.
Retail Windows can only ever be installed on one PC at a time, but it CAN be moved from one PC to another, as long as you uninstall it from the first PC before you install it on the second PC.
So it really depends what package of Windows you have, OEM or Retail.
OEM CANNOT be moved, Retail CAN.