Imac continuosly goes to sleep

Probably due to sleep mode. The computer will power off things such as USB ports. You can disable "Allow the computer to turn off this device to save power" under Power Management tab in Device Manager on each "USB Root Hub". Not sure if there is an equivalent on Mac

re install os after it your problem will be solve

one time check your wifi modem and its signal range

Do you have a wireless remote or keyboard that may be out of batteries?

Either reinstall the lastest combo update or reinstall mac osx

try it to format completely

if you are talking about the apple symbol, I can't be turned off. if you want that off, you need to turn your screen backlight off then illuminate your screen with another light source which is a bad idea.

it just lets light from the backlight out to illuminate it. it is built that way.

This is a problem with your screen caused by excessive use, not turning the computer off or not allowing the display to sleep. More lines will appear if you continue treating it as you have; go into System Preferences --> Energy Saver and change the Display Sleep time to be shorter, like 15 minutes or so and check the "Automatically reduce brightness before display goes to sleep" box. Hopefully it will not get worse or you can live with lines on the screen, otherwise you will need to replace the screen which could cost roughly half or more of the cost of the actual Desktop.

I recommend unchecking 'Put Hard Disk to Sleep When Possible' from Energy Save in the System Preferences. This resolves most hardware related sleep issue.

The long tone and flashing light has to do with your firmware. If your mac boots up, you could try to download the latest firmware update and install it. But more than likely you will need to take this one into the shop.

On the Dell, have look at your network connection setup. ( You don't say what OS you have on it ) There could be a box with a tick in it to automatically connect to nearby wireless networks. Disabling this may help.