When you first switch on your PC do you log in? If you do log in, do you log in as administrator or admin? If you do this is bad because if any nasty stuff comes in through your browser it will install automatically and after that point the Malware owns your PC. If you log in via a user you will always get a dialog box come up to ask you if you want something installed. If something like this pops up and you are not installing anything then you know there is something trying to get admin privileges and install on your system - not good!

If you dont log on when you first turn on your PC then this is even worse again!