Windows Vista 'resumes' after shutdown then crashes

Hi,
I have Windows vista basic on my laptop. Sometimes when i go to start up my laptop, it says 'windows is resuming', like when you have put it in hibernate, when i have previously used shutdown.
then it will get stuck on either a black screen, or on a screen that looks like my desktop, but without the icons (bypassing logon).
it will then turn off completely.
when i then press the power button it starts normally.

it doesnt do this all the time, but quite frequently. i havent installed anything recently that may interfere with anything, and dont start up with any external devices plugged in.

i dont know that much about computers so i dont want to fiddle about in the registry or anything without guidance!
thanks
 
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