To all the PROS out there, HELP!!!

I have an AMD K6-2 450 MHZ system with 8.4 GB HDD, 64 MB Ram and . Whenever I try to install Windows 2000, NT or Linux in my System, My system keeps on rebooting. It loads the OS after presenting me a Dual boot option (the other was Win 98 before and now Win ME). After showing the intial screens in NT or Linux, it starts rebooting. I tried my best to find the problem, but don’t have a clue what’s wrong. While working with Win 98 or Win ME, I never faced any problem.