Win XP freezes with complete keyboard and mouse freeze while playing movies/games?

[Coolmohit]

My new PC
SPECS:
Core i5-750 CPU
2Gb Kingston DDR3 RAM
1Tb WDC WD10EARS Hard drive
Gigabyte H55M-S2H Mobo(Bios F5)
coolermaster 600W power supply
1GB XFX raedon 5770 Graphic card

is behaving strangely.
I have installed Windows XP SP2(3 times)with Microsoft Security Essentials and all other Necessary softwares.Have cleaned Registry nearly 100 times and Updated Drivers every time i start my PC.

Can also Play games for 3-4 Hrs and even longer sometimes but my Pc Hangs suddenly sometimes with complete KEYBOARD and MOUSE freeze.
So,every time it freezes i have to make a Hard Restart.

Now i Formatted again with fresh installation of all above mentioned and fully updated with no games installed.Now i started to play movies but after 4-5hrs again it hangs suddenly with no signs and Complete system freeze(even sound also freezes and Repeats like DERRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR.....like that)

I checked Event manager,but no software errors..at that time.

So, what i have done till now:

1)Checked my Hard disk from Western Digital Disk checking software and it passed with no errors.

2)Checked my RAM 2 times with MemTest86+V4.0
No errors found.

3)Checked my Graphic card stability with Furmark Graphic testing and Benchmarking Software.
Stable also at 100C.

4)Checked connections ,Everything is a Right Place...

But Still My PC hangs someimes nearly abt. 1/2Hr of watching movies and sometimes after 8hrs max.

I tried to install GTA:Episodes from Liberty City.
About 1/2hr of installation PC hangs.
Next time also same.Both these times i was also watching HD movie.

Next time i started installation but no other simultaneous work on Pc and it didn't hang.
But ya later when watching movies and someimes playing Hangs.

I don't understand what the problem is.PLEASE HELP ME....
PLEASEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE EE

I will be Thankful to u.
Please somebody HELP to solve this Problem.