OEM and Pre-installed Copies...
While the following is certainly not unique to Windows 7, this should help shed some light on the topic for those unfamiliar.
For the Purchaser/Installer:
1.) A recent change in the Windows OEM System Builder licensing agreement stipulates that you may only purchase/install an OEM Windows license on a computer you are building yourself for sale to an unrelated third party. Purchase/installation for yourself, a friend, or a family member is prohibited.
2.) OEM licenses cannot be transferred to another computer and all motherboard replacements must be for both the same make and model.
3.) You are obligated to provide all support to the end user. Microsoft does not provide support to end users for OEM licenses, with few exceptions.
4.) OEM licenses may only be purchased for a new system build, not to upgrade an operating system on an existing system.
You should consider your role in the computer development process, your provision of technical support, and the end users intended length of ownership of the computer in the decision-making process. The lower cost of OEM licenses may not be worth it.
For the End User:
1.) The system builder/manufacturer, not Microsoft, is responsible for providing all technical support. Only a few exceptions apply, such as when troubleshooting a Windows Update issue.
2.) You cannot install that copy of Windows, or use that license/product key on any other computer, even if the original computer is no longer in use/existence. In addition, you may only replace the motherboard with one of the exact same model.
3.) It is strongly advised that you burn a set of system recovery CDs/DVDs as soon as possible if the system builder/manufacturer did not provide a set with the computer. (Most have not for a few years now.) You should find the option to burn them in the Start Menu, usually under the Accessories or a manufacturers tools section.
4.) If you did not burn a set of recovery CDs/DVDs, you can usually order them from the system builder/manufacturer for a nominal fee. However, they are only kept in stock for the computers lifecycle, so none may be available more than a year after purchase.
5.) You cannot use a friends recovery CDs/DVDs or retail Windows DVD to repair or reinstall Windows on your computer unless the discs are for the same system builder/manufacturer and make/model of computer.