IMac G5- overuse or ready for 'retirement'

As soon as I start running any program after start up the fan starts going full blast and doesn't stop until I shut it back down. Also, I'm not sure if it is fan related or not, but the computer always emits a buzzing sound as well. I was wondering if this is a symptom of overuse since I am always running several programs and downloading stuff (although the computer starts buzzing even if I have only one program running...), or if its because I need a new fan, or if it's because the computers getting ready to ... retire...

My computer is three to four years old at this point and I'm not really sure what the average life-expectancy for an I-Mac G5 is. Either-way, thank you for your help!

Mac Specs:
OSX v. 10.4.11 1.8 GHz Power PC, 768 MB DDR SDRAM

Have you added any RAM at all? Maybe you could check out that the RAM is being recognised in the System Profiler or even easier, go to the Apple icon and select "About This Mac" from the Menu.

If you've added any 3rd Party RAM then you might need to check that it's seated properly ....