My pc gets shut down automatically and it gives no error .
When i am doing aur watching movie it shut down automatically.
I have also scan my hard disk with updated AVG 8.5 but no virus found.
I also think that there may be issused of heat sink.
Please suggest me as soon as possible

Thanking you.

If there is heat issue then it doesn't shutdown, it just turns off or becomes irresponsive.

Does it shutdowns properly or just turns off ?

My PC used to restart automatically untill I uninstalled Macafee Antivirus and installed AVG.

I don't know exactly what is problem with your system, but the problem can be because of some software or BIOS settings or if your PC doesn't support your OS.