you CAN turn off UAC, but that feature is a good thing to prevent trojan from executing program without us knowing (aka, good thing to be in a n00b PC so that it will not be a trojan farm infecting other ppl's PC).
A good Antivirus is a must, even on any system.
and who the f*** says linux and mac are secure? In term of patching, MAC is the worst OS you can get, as it's slow to detect any loopholes and rectify the issue (like the JAVA + quicktime security loophole which too 6 MONTHS to be rectified). It's just that script kiddy and virus writers prefer Windows as it's the de facto OS in the world.