My laptop wouldn't finish booting up past the memory screen. This has happened to me before and I knew it was an overheating issue.
I unscrewed the panels on the bottom of the laptop and noticed that a filter looked dirty, so I removed the heat sink to get a closer look. I found out that what I thought was a filter was actually a huge clump of dust and hair that covered the opening of the air flow path to the heat sink. I pulled off this dust blanket and brushed away the remaining dust that covered the opening and put the laptop back together.
After turning on the laptop, the first thing that I noticed was the fans were quiet. QUIET! Then they turned off after a minute. They haven't done that in years!
Now the laptop is running quiet again and also running at normal speed. And best of all, it's RUNNING!
I have taken the laptop apart before, but I never noticed the dust blanket blocking the air flow.