It could be an issue with you Netbook or your battery. Let the battery drain all the way to shutdown and see how long it takes. Does it match the specs? If not recharge it and try again. If there is a battery issue it should be evident. If the battery is consistent and good then it might not be the issue. Does the issue happen in "time" with the HD on low brightness. Turn the brightness up while on battery (Fn and Right arrow) and see if the same issue occurs on full brightness.
Then plug in the power and put the brightness on low (Fn Left Arrow) does it happen on power with the brightness low.
Take it back to the shop either way and show them the problem. Maybe they will allow you to try a display battery if they have one and identify if it occurs with a different battery.
BTW is the hardrive access loud? Because on these units they should be barely audible.
Goodluck
