hard drive light? That means your hard drive was shut off. Could be a bad hard drive. Also, it could be a bad graphics driver. My old Dell used to hang up nearly daily and show a rather interesting (though completely unnavigable) pattern of colored lines, due to an outdated graphics driver. This seems a bit unlikely to turn the entire machine off, though; it should just hang up the display. Maybe you should try updating ALL your drivers, and if it still crashes, try a different hard drive.
Guys this has been breaking my head for weeks... i just got a old Sony PCG-F190 running windows XP, the computer runs fine and we use only for the net and a really old game but the computer will shut of randomly sometimes when starting or going online but strangely enough never during the game (starcraft) I've changed the power settings to always on and used speedfan to check the temperature but last time it shut off was way below max (going down from 48C to 46C) also when it shuts off the power light remains on but the pc usage light (the yellow one) shuts off and i have to turn the computer completely off and back on.
Can anyone help me with this one?