I'm using Acer 4637Z, running Windows 7. When I turn on my laptop, it will unexpectedly turn-off at start-up. The hard disk light blinks normally prior to start-up. Virus scan is clean too.
Please advice

Maybe a wiring-problem?
Try booting without a battery, connecting directly to power, maybe your battery has a bad connection?

Sounds like a hardware failure.

Sounds like a hardware failure.

As I mentioned ;)

This could be probably the cause from over heating or boot failure
There's something that's preventing your system to start
In order to solve this, you might want to start a System Recovery
or Format the system if possible

This may be a longshot, but is the cooling fan working? I had a the same problem with one at work and removed the keyboard to find the fan had jammed, after freeing it it booted fine.

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