I have my sister-in-law's computer here....the motherboard is bad and I am tracking down a replacment for it (found one in town for $20...just need to get with her to get the money). I was pulling some of the parts off the dead MOBO and decided to remove the CPU cooling fan so I could really clean the dust out of the CPU heat sink. When I pulled the fan off the heat sink, I was shocked by what I found on the under sides of the fan blades. Not just dust...but dust that had been so compacted that it was like stucco. I had to use my box knife to chip the stuff off the blades!
That got me to thinking about my CPU fan....so I removed it from the heat sink. It wasn't as bad as the first one...but it was not as clean as I thought it would be. Took a bit of work with a stiff toothbrush to get it all removed.
The next time you clean your system, remove the CPU fan from the heat sink...usually they are held on with 4 screws. Check the under sides of the fan blades...you may be shocked by what you find there. And while you have the fan off...really get all the dust and dirt off of and out of the CPU heat sink.