stops every 5-10 seconds after start. and automaticaly starts again. loop continues

hello this is souvik. please help me friends. i've got a new problem. few days ago when started, my computer worked perfectly. but when i rebooted, my computer:
1. either kept on restarting before anything was shown. there was nothing displayed on screen.
2. or it would work normally for 5-10 seconds and start automatically before showing up anything. and automatically start. this loop continued. there was nothing displayed on screen.

but this morning when i started my computer, it got one of the following two problems:
1. Either worked for 5-10 seconds and stopped.. (i mean the cpu fan stopped.) and automatically it started and the same thing. there was nothing displayed on screen. it continued on looping like this.
2. Or after turning my computer on the long beep sound is heard and this continues for time and again without any display until I forcefully turn off or press alt+ctrl+del.ut this problem does'nt occur everytime the computer starts. And sometimes the computer goes on rebooting without displaying anything on monitor. I have come to know about this by seeing the Combo led and the keyboard leds. I've changed my RAMs and tried but the problem persists.

My computer specifications:

Video Card: Intel(R) 82865G Graphics Controller
Sound Card: Realtek AC'97
Motherboard: zebronics 865
HDD: Seagate 80GB
PSU: Mercury 300W SMPS
RAM : two 266Mhz DDR SDRAM
2 D-Link LAN cards
LG combo
1 Floppy drive
Operating System: Win XP SP3 and Linux Mandriva 2009