Startproblems Acer Aspire One A150

Hello,
My daughters Acer Aspire One wont start windows.
The screen goes black and a message comes: Windows wouldnt start because following file is missing or damaged. <Windows-root>\system32\ntoskrn.exe.

How can i get the file into the computer again ?

For about six months ago (computer only one month old) there was a similar problem with a dll file suddenly missing. I talked to Acer support and they couldnt help me, they said that it must be a problem with windows and that i had to reinstall windows. I reinstalled ( and lost 500 of my daughters photos ) windows xp with an older version of xp professional that i had from before. Can anyone help me ore should i order a disc from Acer with xp as it was when it was delivered ?
Can i prevent this from happening in the future ? or is it so unstabile ?