My PC boots normally under light load for nearly 1 hour if I keep my PC OFF for nearly 1 day. It may run smoothly for more than 1 hour but I never checked it. (I can play songs, movie, browse internet simultaneously).If I restart the PC, it freezes. And if I try to boot it again, it doesn’t boot.
I keep it OFF for 1 hour. After 1 hour it runs for sometime and freezes again. Mouse and keyboard also stop working. The only way to restart it to press cabinet button for 5 sec so supply to motherboard get cut off.
So if I want to run it for long time, I need to keep it OFF at least 1 day and I can’t restart it.

I checked RAM. IT is working fine.

My configuration is
AMD phenom x2 550 black edition.
ASUS M2N68-AM Plus motherboard
PSU: Xtech 450-24,Max 450w
GPU: NVIDIA GeForce 7025/nforce 630a

Initial processor temperature is 36C which increases to 48C in 5 mins under zero load.

I have updated my BIOS version from 0701 to 0804.

So what could be the problem??

48c Temp is not something that would freeze the system. There can be many reasons for the problems you have mentioned, I would check the system for following

1) Swollen capacitors on motherbaord (they r bend upwards instead of inwards)
2) Voltage that PS is giving to the system, u can check this from bios
3)IDE Cable, strangely sometimes even faulty ide cable can do things like this, u can just swap the one with dvd