Possible Duplicate:
Can I transfer a Windows 7 license to another computer?
If I have a computer which is physically damaged beyond practical repair, can I legally take the Product Key from that computer and use it to install the same Operating System on a new machine?
That depends, if you have retail version of the OS then you can reinstall it on another system. If it is an OEM version that came with the computer then it is not okay to install it on another PC.
This FAQ page from Microsoft provides an extensive amount of information on OEM licensing.
EDIT: Per Johns request below I'll add that OSX can only be installed on Apple hardware, regardless of whether the OS came with the PC or was purchased retail.
You can have two types of licenses on your OS (and other software/hardware), that is:
OEM
An original equipment manufacturer, or OEM, manufactures products or components that are purchased by a company and retailed under that purchasing company's brand name.[ref 1] OEM refers to the company that originally manufactured the product. When referring to automotive parts, OEM designates a replacement part made by the manufacturer of the original part. source - wikipedia
basicly it means that OEMs are always sold with computers bought in supermarekts - ready bundles and it can't be transfered anywhere. You can only change some parts - not entire pc
BOX or Full
This one gives you a free hand at changing destination computer as many times as you want.
