Windows XP users have that ability, but many don't bother with entering user/passwords at start up. I have a "guest" account on both my XP machines, and only the "Administrator" can install/uninstall/alter the Windows configuration.
But I bet that 97% of Windows users are operating in full Administrator mode.
Virus Smirus. Big whoop. Still unaffected after so many years. Hint, don't click on things you shouldn't. Get anti-virus software and use it. Yawn. The virus crisis is over. The spyware crisis will soon be over and soon spam will be a thing of the past. Whatever the next scam will be will pass like all the rest.
Send users an email with an icon that said "Click on this and it will Kill your dog" and 2% of users that like their dogs will click on it. If you still get viruses on your computer, turn it off and get another job or hobby.
My die-hard-Mac-friend's Dell arrives tomorrow. She had to use the Windows machine at work (and got to enjoy the things she could do). Even after preaching the Mac dogma for years and faithfully buying each new Mac, she quietly ordered the Windows machine for her home and put her Mac away. Couldn't get the software from work to run on her Mac and "man, these PC's are cheap!". And, it works nicely with her iPod.