Problem with our Apple Macs shutting down

it could be a processor problem or a thermal sensor problem is the vents clean on the processor is the fan turning fast, does the fan sound like its on turbo. the pc will shut down if something is getting to hot, there are thermal sensors dotted around the board. these show the temp of each area. even the processor has them inside if you had some software that can read these sensors you will be able to find out what sensor is getting to hot. iven the hard drive or sd drive has these fitted. looks foe some software like system temp , i know they do software for windows but i don't know if its the same for macs but i would of thought they would have a similar software

I assume you mean you want to shut it down if you are expecting the fan to stop. In OS X you need to click the Apple symbol top left and choose "Shut down..." or "Sleep". If you need to force it to shut down (DONT DO THIS ALL THE TIME) then press and hold the power button for more than 5 sec till you hear/see it stop working

Their was a problem of the processor. That's why they are going to shutting down because the processor are overheated. So you need to replace that processor.

You would have to buy a whole new unit, G% processor is not interchangable physically with the intl/

this seems to be a issue with the internal hard drive if it does not pass the logo screen, You might need to have this repaired, call you local apple store make a appointment to bring in your imac. sounds like a hard drive corash correct me if i am wroung I been working on macs for 5 years not to long huh? But I am knowledgeable is why you see the apple icon next to my name. Have the hard drive checked you be glad you did.

Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/edwy_1a56970cf1ea4620

http://imaging.nikon.com/products/imaging/lineup/software/index.htm

You could use virtual box ( unless you want to purchase vmware fusion or parallels ) to run windows or you could use boot camp on one of or both of the macs to run the software

There are several reasons why you could be having this problem. You first need to know a little about the various "flavors" of Apple computers and operating systems.
There are two brands of processors: PowerPC (this is what you have) and Intel (what Apple switched to after your model).
There are also different types of system install disks: Specific Version (this is what would have came with the iMac originally and contains only the drivers for that model). Retail Version contains drivers and "Universal" processor code for all of the supported models and both types of processors.
Intel Macs format their drives differently than PowerPC macs. So, if the used drive was formatted for Intel you wouldn't be able to mount it, if the install disk that was used was Specific Model type rather than Retail type when the system was installed on your used disk, you wouldn't be able to mount it either.
The drive you have must be formatted for PowerPC and system installed from either the Specific Version or Retail install disc in order for you to mount it.  

What drive are you speaking about.
PCs and Macs (assuming you are speaking about a PPC architecture machine-- older-- not the newer dual core Intel processor machines capable of running Windows) run on different code due to the different fundamental architecture of their respective central processing units (CPU's). PC software will not run on Macs and vice versa. Exceptions to this rule would be software written using a platform independent language such as Java (open office is an example of such software).
That said, you can transfer any data that you have on your PC to the drive on your Mac. Unless you have an Intel dual core chip, however, or the software is written in Java or the like, it will not run on your Mac. It will just sit there, nicely stored, waiting for you to transfer it to a machine that can understand it.

A quoted suggestion from my Mac Guru: "They need to turn off the Energy saver preferences for processor cycling and hard drive spin down. They would actually be better off turning them down anyway since the G5 duals run really hot and aren't the most reliable model Apple ever shipped for motherboards failing."

As for the iMac 99, It would be more helpful to know what OS the unit is running. I remember hearing a story about a disgruntled employee putting the 'shut down app' in the start up items folder. This caused the Mac to start as normal and then shut down after the startup was complete. Try starting up the machine with the extensions off. 'Hold down the shift key at startup'