computer keep restarting

hi there is strange problem with my friend's cpu
he brought me his computer for format when i can his cd rom not working i took my cd writer n plug in his computer n on back to check cd rom working or not n when i on computer suddenly my cd writer burned like smoking coming out i off the power take out the cd writer then i took his hard disk plug to my another computer format n install worked fine there after then i put back hard disk to my friend's computer when i on back compuer starting showing loading windows xp but after few sec computer again restart n it's always running like that's can load windows at all n keep restarting i check ram is it because bad ram i change with anothr ram but still same problem then i check vga card also not luck this i ?...
what is the problem it seems power supply also new n working condition ?..

can u help me figure out what's going on where is the problem occuring?..

 

After the BIOS screen start pressing F8 to get the safe mode screen. Towards the bottom of the screen is an option to NOT restart on error. Press this option and it should blue screen on the failure. Write down the error code to use for troubleshooting. Then try booting to safe mode and then try booting VGA only on the screen.