We know a Dell can be different to most 'common' machines- but not that different.
In addition to removing dust a little lubrication may required- on the boss of the fan there is usually a label, carefully lift this up and there is a little depression- a spot of 3 in1 oil or sewing machine oil[ NOT WD40] should to the trick
This applies to other fans too - Case Fans Video Card where fitted .
A little housework here too.
Download free this program - CCleaner
Restart the machine.
Now do this.
Come off line
Go to Start,Programs,Accessories, System tools, and click on De fragmentation Tool.
Once started, if never been carried out previously can be lengthy process, so walk away from the machine and let get on with it.
A mouse movement internet activity can cause it to stop.
Still in trouble?
Come Back for part 2