nikbear, here is a link which gives some possible causes for ntdll.dll problems
http://pcsupport.about.com/od/fixthe...a/ntdlldll.htm
there may be something there that gives you a clue...
also look in the Event viewer log to see if that can narrow it down [Start-Run type in Eventvwr.msc] click on Applications in l-hand window and see what error messages in right. Double click on red cross to see what module has been faulting.