I posted my problem on Yahoo! Answers, "My computer keeps shutting itself down randomly but I can't find any virus. How can I 'save' my computer?", and a nice guy (Stuckart) referred me here to "download some easy software which you can run that will give a log of all the adware, virus, etc. that are hidden on your machine"
I'm wondering if any of you here knows what easy software he's talking about, and if it really works, or you know a better solution to this problem.