Installation problem

Hello,
let me tell you my problem few days back i have installed windows xp sp2 it work very well then on the next day i install sp 3 system works well but within few moments i tried to open one of my word doc file and suddenly my system got restarted continuously..after that i have formatted my drive c: again and tried to install fresh copy of windows xp sp 2 the installation started but after formatting system copied all files to the hard disc and a prompt come that your computer will reboot in 15 seconds... and after that it stuck and cannot continue the installation process..
to install the window what i have to do...

Please help me to find out the solution as soon as possible.....I m waiting for your reply


Thanx