Reformatted drive, Win XP pro , upgraded to SP3 and installed AVG 9.0. System worked fine for about a week, then all of a sudden won't boot. I replaced memory module and replaced HD. Still no boot although all 3 green lights lit for about 2-3 seconds when I press the start button then dies.  I am stuck now, I can't reformat again. Tried hooking up external DVD drive with XP cd loaded. still no go. HELP!