Why is it that the Windows 7 hangs up, when I turn on the PC. Â It will hang up and then shut down in a few seconds or in a few minutes even if it has a 500GB HR? DD, 2GBDD?
You might have run or used your system beyond the capacity of your hardware and also you're computer freezes when you run a number of applications at once. The RAM of your computer system, Â cannot hold the number of applications at the same time.
You can also try to update your drivers.  Go to the website of Microsoft for any new updates, it  is best that all your hardware and software devices are compatible to each other.
The other reason why it freezes might be because of the numbers of error registries in your Window system, that prevents to transmit information. The registry of your computer holds in all the information that your personal computer uses to  transmit in all your devices either software or hardware. Make sure that your registry is not full off errors. So clear up all the corrupted and error files in your registry, this will keep you computer from any system breakdown.
