Okay, a bit of a collection of the important points of the thread to help you, man.
First: Anyone who's directly attacking you, ignore. It's ridiculous and unneeded.
That said, you do not have a virus. Like many have said, a trojan is entirely possible and possibly likely if you were installing some shady things. You may not realise they're shady, but if you didn't know they were reputable, they're probably not a good idea. There are no viruses for Mac OS X, meaning no downloads can hide themselves, install themselves, and replicate themselves, unless you the admin give them your root password.
Two: Don't blame yourself, or give up. These things can happen really easy, and even when you're being careful, no one is 100% foolproof. "Hey, this looks like a neat addition to my quicktime, let's download it." can be very tempting. I'd recommend what other's have and just reinstall Mac OS X and give it another go. Keep your Windows box and your Macbook with you, come on here, look through the Macrumors cool programs lists and guides, customise a bit, and just do stuff. After you're pretty happy, spend a while just doing things normal without much downloading and stuff, and see what you think then. I'm sure most here would agree when I say that Mac OS X is a wonderful Operating System.
Third: Don't get too stressed out, give it another go, and if you decide it's not for you, then who cares? Just go back to Windows, because as long as you're using something that helps your workflow and isn't too stressful, what does it matter what it is? Linux, Sun, Windows, OS X, just use whatever you like best.
On a side note, though, how the hell does a Macbook cost $3000 in Australia?! That's like 14-1800 more than in the U.S...