Need help for PC problem

[arthur5552000]

For last 1.5 months I am having this problem ,it will be helpful if someone can tell the probable root cause of this.

Whenever I leave my PC in idle mode for long hours like downloading torrents for whole night or running data recovery wizard software which takes almost 36 hours for scanning a 500GB drive; my PC hangs after 3-4 hours.Monitor just goes black. I have changed hard disk and reloaded XP, but no improvement.When I turn PC off and then again turn on it wont boot. I don't even hear that POST beep sound . After trying this for 10-15 times suddenly I will get the beep sound and PC will start booting. I urgently need to recover data from one of my crashed HDD, but can never get my PC running continuously for 40 hours which it takes the program to completely scan the disk. I will come back office and find my screen is black,monitor power indication light blinking.
My config is :

Intel Core 2 Duo E4500
ASUS P5B-MX/WiFi-AP
ZION DDR2 2GB
Seagate Barracuda 500 GB