Computer says 'Restart'!

I have been facing this mysterious and debilitating problem... I don't think there is a cure.. I have looked.. and found! and Installed!!...and Failed :( 

My computer has been afflicted by - The Restart Problem. My computer just restarts whenever it wants to, whenever it wants to (um..I couldn't write 'wherever' coz its a desktop computer) for e.g. when I am writing a long important email (& have not saved a draft), or when am in the middle of an online conversation with my manager or just re-worked on a document and made a million changes and am about to hit 'Save'...

I am sure you understand the mental agony and torture that my computer's affliction causes me...coz my computer doesn't seem to mind the restarting, the non-saved mails and documents or the mental torture for that matter...It just sits there, mocking me and my frantic antics everytime it restarts twice in a row - second restart being just after I have just logged in after the first re-start - aaarrggghhhh!

I don't know whether it is a Virus coz the scans can't come up with anything; or is it updates being installed automatically even though I have changed the settings for that. I am at my wit's end and so is the IT guy!

To quote two lines from the poem of an anonymous but awesome poet,

"The more I try to forget it, the more it is fixed on my mind,

I guess a solution to this problem, I won't ever find"