Vista is eating up my C drive

Microsoft releases updates for Vista so quickly that my 40G C drive was soon crammed with system programs. So I forbade automatic update for a while. Three days ago, Sean, my housemate, knocked on my door and told me that I probably got certain spywares on my computer, which allowed hundreds of open ports on it and caused lagging in our house's internet. It sounds formidable... So I turned on the updates again. Following that, within one day, my C drive turned red again. I've no idea what I can do... Should I install a linux system on my computer?? Or go back to XP???