Windows has recovered from unexpected shutdown

Hi,
My computer shuts down and reboots randomly and I keep receiving this error.
Windows Error
Windows has recovered from an unexpected shutdown.
Windows can check online for a solution to the problem.
Most of the time, when I left my PC open with its screen saver on, for a few minutes, it will instantly freeze and the error from Microsoft Windows will appear. It has options to check for online solutions but I can't even move the mouse so all I can do is to force close the whole computer by pressing the power button on its CPU.
This happens in no time, and it is totally annoying. What I did to lessen the error from re-appearing is, every time I force close it, then restart it, I'll make sure I will shut it down again properly. Anyway, I don't think that by doing that process helps my computer to stop this error. I want to know if my operating system is corrupted or some boot files of my Windows Vista or the hard disk itself is corrupted. Performing a startup repair by using the installation disk will took me an hour to finish executing.
My laptop is an Acer Aspire 4730z with Windows Vista Ultimate. How can I get rid of this error?
Please send me some help.
Many thanks.
