My Compaq Presario Is Driving Me Nuts

My Compaq Presario (SR1 299AN) is so LOUD! The computer fans often
turn on, and the sound can be overwhelming. I'm not sure why, but it
particularly goes nuts when I open microsoft word and type manually -
and I'm forced to type because the racket drowns out dragon naturally
speaking ( I have RSI and would prefer to use speech recognition).

And yes, I often clean them to remove dust, etc. So my question is:
what can i do to fix this? Can I replace the fans with something less
noisy and/or constant, etc?

thanks in advance