The machine is a HP pavilion - celeron 1.7 ghz. I added a DVD drive and more RAM and turned it on. After post i got a "Operating System not found" message on a black screen! I turned it off, waited a while and turned it back on and this time it loaded windows 2000 Pro. I tried out the dvd drive and checked the RAM, then switched it off to put back the panels and screws. I turn it on and "Operating System not found" comes up again. I take off the panel and try to boot up but same message shows up everytime! What does that mean? How do I resolve this?