Can anyone help me with a prob?

Bondi blue iMac 350 with 192 Ram running 9.0.4 (keep forgetting to upgrade) starts the day OK but progressively slows down and eventually hangs. Slow on reboot also. Happens across multiple apps: Appleworks, MYOB, Outlook. Finder too. Things I have checked:

1. 7g HDD, only 1G used so not likely fragging (mind you it has been in service 3 years)
2. Plenty of memory allocated to said apps
3. Desktop rebuilt several times
4. 4 meg HD cache
5. Same with virtual memory on or off
6. Peripherals (HP inkjet printer, USB Zip 1.0) removed does not change it.

I don't think there is a fan in this machine or if so it has never been known to have worked - I thought it might be overheating.

does this ring bells for anyone?

Thanx, David