When I am using my Dimension 8300 the screen will randomly just go blank and the computer goes on standby.  Lots of times when I try to reboot it right after that happens it will come on for a second then shutdown automatically.  After I let it sit a while it will boot up but has an error message about a thermal event.  I have had this computer for more than a year and never had this problem before.  I just installed a new western digital hard drive and a new copy of Windows Xp w/ service pack 2 about a month or two before this happened.  The first time I experienced this problem I was playing Warcraft 3 if that has any concerns with this issue.  Now it does it not only when I am playing Warcraft but when the computer is sitting idle at the desktop.