A blocked fan is the most likely cause of it slowing down. There are others!
Some software that you might load to "try" may have set itself up to run every time you power on. You can check this:
Click on the globe bottom left, and in the "search" box type
msconfig and press Enter.
Click on the "startup" tab and look to see what's there. If you don't recognise any items there, untick them, then "Apply" and restart the machine. Check that they have stayed unticked, and see if it works any better.
Skype and iTunes are very good at adding themselves back in
