Does it chime? if not, you might have a logic board failure, or your volume is low. reset the PRAM by pressing "Command" + "option" + "P" + "R" at the same time immediately after you press the power button. keep it presed until you hear the chime twice. if you re still holding aftr 30 seconds, stop, you might have a faulty logic board. If that didn't ix it, hook up a display to the vga connector at the back to see if you get the same thing? if it works correctly on the external, then your issue is either the display connector to the CRT or the CRT itself. i can get out one of my imac G3's and try and get guide made for you. if it's giving you the same thing, then it could be the logic board, the RAM, the hard drive or the software. Try booting up pressing the opion key. if the botloader comes up then your logic board and RAM should be ok. Try booting to your operating system disk and try reinstalling the OS. if it fails, replace your harddrive and try again. If you need a guide for that, tell me, and i'll...

take a look at this guide. Electronics Water Damage it is very descriptive of how to clean the different parts. I do recomment trying the rice trick (Step 8).

I would contact Apple Customer Relations and tell them that during the repair of the motherboard for the nVidia GPU issue, that the tech damaged the left IO board and they didn't replace it. As for if there is something that can be replaced or resoldered in place, i don't know the part enough to tell you. Somebody else here will be able to tell you exactly what part on the left IO board to look at. Personally, I would just replace the left IO board as you will at least know that it will work as you have already tested that way.

if you have a bluetooth module, you can try using a bluetooth mouse and keyboard. your imac should ask you pair a keyboard and mouse if you have the bluetooth installed. as for the damage, the ports are part of the logic board. i have never seen the come seperate. i would have to say

please see this guide. PowerBook G4 Aluminum 12" 1-1.5 GHz DC-In Board Replacement

first thing to check is your video card. check to make sure that it's properly seated in it's connector. I know that sounds strange, but i've seen this before. to check the power supply, remove the cap covering the connector. disconnect the connector from the logic board. plug in your power supply, do not turn on your power mac. using a volt meter, check the voltage between pin 1 and pin 23. you should get 5 v. if you get 5 V, then your power supply is good and your logic board is the most likely point of failure. if you do not get 5 V, the nyour power supply would be the most likely point of failure. i'm having a hard time posting an image to my answer, so i'll try to be descriptive. Pin 1 is on the oposite side from the clip on the short side. pin 23 is on the same side as the clip, second from the end on the long side.

What operating system are you installing? if you are installing Windows XP with SP2 or older, please make sure to enter your BIOS (f10 on Dells i beleive) and set the SATA mode to "compatible" or "IDE" mode. AHCI or RAID modes will not work correclty with Windows XP SP2 or older when reinstalling from OEM discs unless the drivers are properly loaded. If it's Windows XP SP3, Windows Vista and Windows 7, then you may have a fauly motherboard. Do you get the same "Hard Drive Error" when doing the tests with the new hard drive?

i would replace the hard drive as soon as i can. you never know when the failure will occur and you will most likely loose an important document, especially if you are working on it at the time of the failure. to be absolutely certain that something is happening, use a software like Tech Tools to read the SMART parameters on your drive, you should see if there is a problem based on the results.

PRAM contains many system settings including the volume, some display settings, the time zone, etc... see this article from Apple for the full details http://support.apple.com/kb/ht1242

sounds to me like it's your inverter cable. see this guide for replacing it. MacBook Core 2 Duo Inverter Cable Replacement as for teh cable itself, please provide us with the last 3 digits of your serial number and we will be able to provide you with the exact part number as there are a few versions of the cable.