The first question has to be: what operating system are you using, and when did you last do a fresh install? The symptoms you describe are often associated with a PC that, over time, has just got filled up with all manner of crud. I also suspect that a million programs may be automatically starting up when you boot the PC: that would explain the symptoms too. You can find out what's starting up by going to msconfig. The fact that, other than it being incredibly slow and unresponsive, it still works tells me it's unlikely (but not impossible) to be some form of malware.
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