Windows 7 - why my computer's fan making large noise at start up time? A week

A week ago everything was running fine 3,4 days ago i notice that it is making much more noise than it should. It seems that it is CPU's fan's noise.Most probably dust is responsible for this.there is a lot of dust inside my CPU. This may hamper hardware badly(as i have seen in some websites). What should i do? can i solve this by my self or i need to go to computer repair shop.
one argument that this sound is not of fan is that, If it is sound of fan then it should start blowing very fast with noise after opening computer for long time since it blows when processor warms. But one can say that fan is not in well working state so that's why it is making this much sound, After all these are my arguments. I may wrong. what you think is possible reason for this problem.
thanks to all those who reply.

For waht amount of time is it load? Lots of Motherboards start up with the fan at full speed until the CPU temperature can be evaluated. However, that is for a very brief period of time.

If you have lots of dust in the heat sink cooling fins, then it may take a while for the processor temp to stabilize. Best to blow the air out of the fins.

what is that time. some seconds or minutes?
I am going to clean my CPU's dust today.after that I will tell rest story.

It's possible (and not that uncommon) that one or more fans are developing bad bearings and will eventually just seize up and quit working. Fortunately fans are pretty cheap and easy to replace, with the exception of some CPU fans maybe.
You might go into your BIOS and see if it has a monitoring feature showing you all the connected fans and their RPMs to see if one is going particularly slow, and if it's having a noticeable effect on CPU and motherboard temperatures.
If the BIOS doesn't monitor these things, Windows software such as CoreTemp will.