I've had my NB550D for just under a month. I've upgraded the memory to 2Gb and the Op System to Home Premium, but nothing else. It's on auto update re security issues and I've stayed with the McAfee protection that was pre-installed (and paid for a year).
Everything fine until a couple of days ago when switching on, I just got black screen. Could hear fan and disk though. Eventually pressed the on/off button, nothing changed, waited a bit, pressed it again and it turned off. When I pressed again it came on as normal. Used it several times over last couple of days and was fine. Now today I switch on and black screen again, and can hear fan. When I eventually pressed switch, I could hear disk whirr but still just black screen. Since then have tried pressing on/of switch occasionally but no change at all - can't even get it to switch off!
I read in another forum that the answer is to take the battery out (having already taken the mains out) so it'll die and then press and hold the power button for 30 - 60 seconds. I had nothing urgent to do so let it die naturally when the battery ran out rather than take the battery out when it was on. I then held the power button for 60 seconds, plugged it into the mains and switched on again. It's started fine now but of course I don't know a) whether that's because I held the power button in for 60 seconds and b) whether same thing's going to happen again, so if anybody does have experience of this behaviour, I'd be grateful for comment.