Ok so it isn't Panther 10.3.x. I was wondering, is there any reason why you haven't updated your system to the latest version? The latest version is 10.4.9 with some security plugs here and there along the way...
Ok so first, I would run Apple's Disk Utility to repair disk permissions 'cause I think the system could be messed up a lot. And then, I would download
MainMenu, use it to clean out all the caches and use the rebuild commands to update the Locate Database and LaunchServices. You will need to restart at some point. Do not worry if you feel the iMac is taking longer than usual as the system will be rebuilding from scratch all its cache files.
Am I mistaken in thinking that you probably haven't run the maintenance tasks for a while? From now on, you should use MainMenu every day to run the daily maintenance tasks, choose a day for the weekly task and once a month run the monthly task. This will make sure that problems like these will be minimized in the future and it really doesn't take long or much of your time anyway.
And if you plan on updating to 10.4.9, a few precautions: you need to unplug all the peripherals, except keyboard, mouse and the modem cable, from your iMac, restart and launch System Preferences so you can use Software Update.
During the update, do not stop your download or the installation and do not run any other application: just let the updater do its thing without hindrance and you should be fine.
Good luck and let us know how this goes.
