I have the answer to this question. After 28 years of working with average users I find the answer to be obvious.
1. The average user only learns the function he needs to get his daily tasks done. He is ignorant of 99% of the operating system.
2. The average user doesn't know if his computer has a firewall or anti-virus protection. He has no idea that virus data files must be updated and/or his update license has expired.
3. The average user will click on anything and reflexively clicks yes at every opportunity.
4. The virus writing crowd concentrates on XP because most users use XP. The same is true for malware and adware/crapware writers. There is little point in writing programs that will never run.
Linux is a good desktop if you don't want to run the thousands (millions?) of programs that are available for the PC. Linux is reasonably fast, almost free to operate, and can do most office tasks adequately. The office suite software for Linux is acceptable, but not exceptional.
Linux is ready for the desktop, but not ready for the average user who doesn't want to learn anything and wants to play Doom and Grand Theft Auto.
I use Linux, but have to switch to my PC to run Exchange. Perhaps there is a kluge for that problem, but I don't know it yet.
The other thing you have to remember about "average" users is that, on average, they are dumber than you.
Windows has to be dumbed because - let's face it - 50% of all people are dumber than average.
Domandred: $ ./configure $ make depend && make $ sudo make install
Expecting that people who are functionally illiterate could type that string of characters without introducing typographical errors - much less that they could understand what they were typing, or why they were typing it - is ludicrous.
It's like asking the average American to get up off his fat @ss, walk over to his TV, unscrew the back panel, and begin testing resistors or capacitors for replacement.
H3ll, the average American can't even set the time on his VCR.