Microsoft used the "WGA" program to turn off people's legal copy's of Windows XP....because they "saw other computers on the internet with the same CD Key".....Who knew they could see all computers on the internet.
They can't see all computers on the internet.
It is very straight forward... if you have a Microsoft based operating system that is still supported by Microsoft you can set how often your computer checks for updates... you can make it every time you connect to the internet or as specific times of the day, or you can make it prompt you to update or you can do it fully manually and do it when you choose.
It doesn't matter which way you do it to update your operating system it needs to check what you have installed with what is available. If you install the genuine advantage software and run it it will do a quick check to see if you have a valid CD key. If not then the next time you choose to update your operating system it will squeal to Microsoft that there is a problem so instead of downloading updates it downloads a kill switch to turn off your software. Now of course because that software is actually the operating system then your computer will suddenly not be able to find windows.
If you have more than one operating system on your computer then it will likely default to the other OS, but most people just have the one OS so their operating system stops booting up.
They haven't changed anything in BIOS... they have just disabled their OS on your computer so when your computer goes through its normal boot up sequence the bit after it does the hardware checks it is supposed to look for an OS and if your operating system is disabled it ignores it and looks for another one. When it doesn't find one it is stuck and tells you it can't find and operating system.
If you get a USB memory stick and leave it plugged in when booting up you will get a similar message because on newer computers it will look on the USB key for an operating system but when it doesn't find one it doesn't know to look on the hard drive next and stops the start up process. (In which case you can turn the computer off and remove the USB Key from the USB port and turn the computer back on and it should boot up normally.)
Of course it also means you can install a light operating system like one of the Linux versions designed for netbooks on a USB key and put your operating system and lots of software on it... there is lots of good stuff at portableapps.com that is all free and can all be put on a memory stick... the portable version of notepad is awesome BTW, but you can include open office etc on the stick and put all your email on it and it all stays on the stick... excellent for using in cyber cafes or at work to keep your privacy... though for important stuff backups are essential.