My computer has slowed to a dead crawl.

In the past few days, my computer has slowed to a dead crawl. I have defragged, run virus scans, malware scans, and nothing has helped. If my system is to be believed, I do not have a virus or spyware. So, what has happened, and what can I do about it? For example, when I went to register so that I could ask this question, it took me 25 minutes.