Thanks for your answers, I have decided to open the computer and do a proper dust removal with compressed air. However, this did the trick only for a short period of time. After a month, the Mac is again wants to start flying, despite the fact, I have already change the hard drive and add 2gb RAM. I think this should be working properly, but unfortunately nothing changed, except the few percentage speed enhancement.
Too bad, the computer is not in warranty anymore, because this is becoming a bigger problem in the library more and more.