Windows all the way! Mac is so bad the graphics on it make you go to sleep, sooooo round. Never actually used Linux but I heard it was pretty good while UNIX is a Mainframe operating system and I don't think I'm gonna use one of those for a while :P
Well as the other people said defragmenting and adding memory are the best ways to speed up your comp. Although you could also delete your old and I mean old work.
Sorry to say this but you have major spyware. Go to the Microsoft Security Homepage and download the anti-spyware software. Once you have downloaded it run a system scan and that should remove probably half the processes (this happened to me before so I know how to fix it :D) After this run a virus scan and that should probably delete a few processes. If this doesn't work and it is still really bugging you may have to resort to deleteing your hard drive and installing everything again. Good Luck with fixing your computer :!:
Well you should download Microsoft Antispyware Beta1 from http://www.microsoft.com that should get rid of the spyware then use Norton AntiVirus System Scan as the other people have been telling you. This should remove your computer of all major threats and after that just stay on the safe side. Don't click any links that you don't know where they go (I know this is hard just try) and well run regular scans. You should be alright!
Lol i can't believe its number 1 in Australia as well. It sounded cool when I first heard it but then it got sooo annoying. Also the payment is rediculous, you have to pay $5 a month even if you delete it from your handset!