Shoutbox

I dont think its an hardware problem..

Do you have any norton product?

It may seem stupid but i never had good experience with norton products ( firewall and antivirus ) they would stop my system from loading correctly and i had to restart just like you..
In this new computer, i had almost the same problem with the antivirus but it instaed of stopping would take ages to load.

Or it could be a virus, check what are the things that startup with your computer.