Comptuer started randomly shutting down.

I'm running a Dell XPS 702x, and I've own it for 2 years without much of a problem.

Until recently,

The issue is that the computer started randomly shutting down without warning. It happens a lot when I play games like TF2, and I've never gotten this issue before.

Now my computer wont even fire up unless I'm holding the Start key.

Its running Windows 7 64-bit

I have no clue why this is happening, or how to fix it.

A lot of times, computers randomly shut down when playing games, because either the CPU has improper cooling (50% of the time) or the video card is failing (20% of the time) or other issues (30% of the time).