From Microsoft's OEM Software Licensing: Rules & Restrictions document:
License Transfers*
OEM Software may NOT be transferred to another machine. Even if the original laptop, PC or Server is no longer in use, or if the software is removed from the original hardware, OEM licenses are tied to the device on which the software is first installed.
Transferring Licenses
OEM Software may NOT be transferred to another machine.
• Even if the original laptop, PC or server is no longer in use, or if the software is removed from the original hardware, the OEM licenses are tied to the device on which the software is first installed.
• As long as the license and device remain together, there is no limit to the number of times they may be transferred from one user to another.
• When transferring a PC to a new end user, the software media, manuals (if applicable), and Certificate of Authenticity label should be included. It is also advisable to include the original purchase invoice or receipt. The original end user cannot keep any copies of the software.