Could be any one of several things.
Hard drive problems,registry problems,power supply fan,CPU fan,conflict with apps. Have you made any changes prior to this problem like installing a new program?Opened any attachments(virus possible)?How are you system resources?Are you starting every program at boot up? It could be as simple as your on/off switch(probably not). Like some of the cattle questions need more info.
What does it do when it starts and quits?
What is your OS,virus protection,power supply(home not system)
There was a virus/trojan about 3-4 yrs ago that shut down the system at boot-up, but if you have downloaded the latest patches/fixes I don't think that will be it.
pm me and we'll see what we can finger out.