LightBlue25

hi

i am facing a strange problem in my computer, i am trying to install the winxp but the computer keeps turning off on reset or when i make changes in the BIOS and press F10 and save and reset, or when finishing the first part in windows installation when it tells "if there is floppy in the drive remove it" and it start count down until it resets the computer, and in this part sometimes it turned off the computer and sometimes it didn`t,
or even when i use the CTRL+ALT+DEL from the dos it turns off the computer, and sometimes in this part it dosen`t turns off the computer.
and the thing is, that when it turns off the keyborad Num Lock key light remains on.

i don`t know what`s is the problem.