My acer aspire 6920g will shut down automaticly when i want to log in to my system, after the boot menu the cooler is turning off for 1 second.

There are many reasons to occur this problem Battery You computer's battery may be experiencing problems if your computer boots up but shuts down shortly after booting up. The battery may not be holding a charge properly because the battery is old, the battery charger may be damaged or the DC jack on your laptop may not function properly. If your laptop's battery is more than a few years old, it's likely that the battery is losing the ability to fully recharge and may need to be replaced or plugged in all of the time. If your charger is damaged, you can replace the charger with a new one from the manufacturer. If the power connector jack on your laptop isn't working properly, the plug for your charger will keep slipping out. This won't allow the battery to recharge and you'll need to have it repaired to stop the problem.
Hardware Your laptop may experience hardware conflicts that cause it to shut down prematurely. It's not easy to remove a laptop's internal hardware, but you can easily remove any external hard drives, cameras or USB devices that are connected to your laptop. After your computer boots successfully without these devices, add them back one at a time to your system to see which device is problematic. You can stop using the device or you can uninstall the device that's causing problems and try to reinstall the device drivers.
Heat Excessive heat from a malfunctioning fan, CPU, memory, video card or hard drive may cause your computer to shut down. You can replace the component that's overheating or replace a damaged fan to reduce your laptop's temperature. If the components inside your laptop aren't failing, you can use a laptop cooler to reduce the overall temperature of your laptop. You can also help your laptop maintain a lower temperature by cleaning it regularly with compressed air to increase airflow and reduce the buildup of dust and dirt.
Software Software conflicts can often cause your laptop to crash shortly after booting. Software conflicts can arise for a number of reasons, but it's common for multiple virus scanners that open at startup to cause conflicts. If you have a firewall installed, it may be denying startup services that your computer needs to operate. You can remove any unnecessary startup programs from the Windows System Configuration tool. This tool can be accessed from the Control Panel

Hello
i am having issues with my aspire 6920G
when pluged, the notebook shuts down when windows is loading
even when trying to install a fresh copy.
so it shuts down and after a few secs. starts again...

with battery, no problems at all.

you can also start it in safe mode with no problems even when pluged

does any whizz know what the problem is and how to bypass it?

When the CPU overheats then it will shut down (i.e. thermaloverload) and won't boot up again until it has cooled down. If this is theproblem is not fixed then the CPU will eventually fail.
A faulty fan causes the problem or the fan and heat sink assembly are cloggedwith dust.
Check the CPU fan to see if it spins freely, if not replace the fan.
If the fan and heat sink is clogged with dust the remove the dust and make surethe vents are clear and dust free. Use a can of compressed air to do this.