If people would get the hell off of ancient browsers and operating systems (like Win98 and IE6), issues like this would come to a damn near dead halt (as well as 90% of the spam and botnets on the net would stop).
Aside from OS and browser security issues from being out of date, nothing and I mean NOTHING can just install itself to your computer. Usually this is caused by some stupid user doing something, well, stupid. Generally stuff like when someone who doesn't really understand what they're doing just decides that because some random popup says, "Your computer is infected, click here and download X to clean it!" that it's a good idea to follow what it says and download said *spyware* to stop that pesky spyware or virus that's on their computer.
Then when it's all said and done, people get frustrated and decide that the only way to fix it is to reformat their computer... which is also equally as stupid now days. The fix I generally find that works better than reformatting is, "Install a new OS or get a new computer".