The processor's fan is making that annoying noise more than ever. Why?

Whenever I access a Flash made website it sounds like it is raising speed, whenever I open a program it does, even with notepad, and sometimes when I'm using Word, it starts again whenever I start typing, that's pretty annoying, but I don't know if it means that I have to buy a new one.

Here is my PC description:

Motherboard Intel D101GGC

Processor Pentium D 2.66GHz FSB 533Mhz

768MB DDR Ram (Free ~420MB)

XFX 6200LE 128MB Video RAM PCI-E

Western Digital 80GB IDE Hard Disk

Windows XP Professional Service Pack 2

Runtime description:

Normal Windows processes, besides that the only software running in background is the Firewall/Antivirus Norton Internet Security 2007.

This system was built at once and all hardware have almost a year and a half. Hardware configuration has never been altered, niether in the OS nor in the BIOS, no overclocking, no hacking, nothing. All software is orginal. This normally happened when High processing was required, but now it happens at any time

The processor's fan is making that annoying noise more than ever. Why?nortonantivirus

The cpu fan is probably clogged with dust and having to work too hard to try to keep the cpu cooled.

Buy an aerosol can of compressed air for cleaning computers. It must be one that contains no liquid. There are several brands such as CleanSafe Dust Remover from Wal-Marts and Falcon Dust-Off to name a couple.