Monitor, keyboard and mouse not working

Formatted hard drive, reinstalled Win XP for Home and new keyboard and new mouse.  Monitor light blinks green - - have "tested" it by unhooking video cable, monitor says it's working fine, but screen is blank after regular bootup.  No action from keyboard - - when machine is turned on, keyboard lights flash green, then no lights at all, including the num lock key.  Mouse doesn't seem to work, but since monitor is blank, it's hard to tell.  Was having trouble with this issue before re-format and reinstall of XP - - three days after that, I was copying data from CD drive to hard disk, walked away, and when I returned it was not working.  Is this an issue with the video card, or am I just not "awakening" the machine correctly?  System is an XPST-500, 256MB ram.