NOTE: In the US at least, during the XP days there were plenty of loopholes in the license that made it seem OK to buy System Builder for home use (see the Ed Bott artlce I linked). I myself did so during that time and advocated doing so. Around the release of Vista SP1 the license was rewritten to include the language I am referring to that requires you to resell the system to an unrelated third party in order to accept the license. This seems designed to close the earlier loopholes. (Previously many of the license restrictions of the license only kicked in upon resale of the system.)
The system builder edition is technically absolutely identical with a full license except you are not entitled to free 90 days of support which is immaterial to most people anyway who still use XP.
This is often said, but is simply not true.
There are subtle differences between retail and System Builder/OEM keys, e.g.: you cannot use the System Builder disc to perform an upgrade install (say in-place from Vista 32 to 7 32), System Builder requires you to do a clean install. If you reinstall retail on new hardware it will usually activate online just fine while OEM will force you to the phone system.
The sheer number of permutations and complications involved with the fine details of Windows licensing (at least in the US) aren't (IMHO) worth the hassle.
If you can't afford a full packaged product copy of Windows, but are willing to ignore the license restrictions that come with OEM/System Builder to get a cheaper price, why not just ignore the license restrictions of the retail upgrade version instead? (which at least is designed for licensing by end users directly and is often available for a similar price as the OEM/System Builder version) You can work around the limitations of the installer that "requires" a previous install pretty trivially following any one of the guides around the 'net. The license you get is equally functional and valid as the SB/OEM one.
So what is the ultimate appeal of System Builder/OEM editions for the home user in the US? I just don't get it. OEM places restrictions on you that are not imposed by the retail upgrade version and sometimes you can even get retail upgrades cheaper than OEM.
In the OP's case, one could easily rationalize that (if the laptop is really to be retired), that license could be used to qualify for the retail upgrade pricing and the new W7 license clean installed on the Mac.
B