My laptop won't boot. It doesn't go through post and I get the motherboard progress/activity bar , then it just sits there. Nothing happening. But if I press the DEL key it continues the booting and windows starts.After that some key stroke sound will come.If I press anykey, the sound disappears,but if I start to type something ,letter "a" is missing(key "a" not working.what it could be?please help me....