Oops, OT is my middle name! Grrrrrr. Ok, here's the deal. Every time, I deviate from the topic.... well, forget it. I won't bore you any further. Well, now once you have opened up your computer, removed all the various cards and the IDE connectors, carefully remove the CPU fan's cable, and then remove the CPU fan from atop the CPU [Do this carefully. If you screw up, don't coming running to me].
Now, you have 2 options. One is the dry way, and the other is the wet one. I used a hybrid. Here's what I did. I removed the motor along with the fan blades, and removed the dust from then using my vacuum cleaner, in high power blow mode. To remove the dust which was stuck hard on the heat dissipating alloy, I dipped in in warmish water, and then washed it under moderate pressure running cool water. This ensures that after the dust has become loose, we can wash it away using hydraulic power. Then, wait for it to dry. In the mean time, we can get down to blowing out the dust from on top the mother board [using a vacuum cleaner again]. Once that is done, check if the alloy is dry enough to be put back. If it is then, attach the fan to it, put it back atop the CPU, make the various connections, put on the casing and you're ready to use an overhauled computer! Now, isn't that simple :-)