On the technical side, Microsoft doesn't track where you've installed it, they don't keep a record of your IP or anything like that. When you activate Windows, it generates a hash sequence (code) that it sends to Microsoft, which is based on certain types of hardware you have installed and identifies the computer (only for use with Activation of course). All Microsoft keeps a record of is that your CD key was activated already, and I think a copy of that hash code.
According to the license terms for OEM Windows, you can only install it on one complete computer system, and you cannot transfer it to another system once it's installed. If you upgrade your system by changing out a mainboard, your license is also no longer valid, unless the change was because the hardware failed and the new board was the closest equivalent the OEM could supply. Since you are effectively your own OEM (the terms of the license are somewhat vague and can be read to say anybody who builds a computer is an OEM), you could replace a mainboard with a better one IF the old one died, but you can't just upgrade for the hell of it.
The only difference between OEM and Retail are the license terms; the OEM license is more restrictive because it's intended to be included with pre-built systems, and is sold at a cheaper price than retail so that OEMs can sell machines with Windows pre-installed without it increasing the cost of the machine as much. The trade-off for the cheaper price is the terms such as not allowing you to use the license on any other machine.
Probably the majority of people who buy Windows OEM versions from places like Newegg violate the exact terms of this buy continuing to use their OEM license when they build a new machine. Microsoft does not bother with trying to track that sort of thing, yet, as it would mean a tremendous amount of time and money finding people who do it, and trying to prosecute them, and would result in the terms of their license agreements being tried in court and possibly being invalidated.
Installing an OEM or retail version on more than one machine is just plain against the license terms and there is nothing that could be argued against that.
If you gave your machine to your brother, and didn't wipe the OS, then activation wouldn't be needed again, and that's completely within the license terms. If you wipe the OS and reinstall, activation will be needed again, but it doesn't matter where it's located when you activate, and again it's still legal. Depending on when it's activated, you may have to make a phone call to Microsoft to give them a code generated by Windows, as the automatic activation may detect that it was activated before. It's a simple process and they rarely hassle anybody about it.
You cannot legally give your brother the computer with the OEM license and also install with the same license on your own new machine.
More information:
http://forums.anandtech.com/messageview...atid=34&threadid=1904966&enterthread=y
http://forums.anandtech.com/messageview...atid=33&threadid=1876863&enterthread=y