Recently, I've been having this problem.

I'll be doing anything, surfing the web, playing a game, or maybe idling my computer, and it will either a) shut down for no reason, or sometimes b) freeze.
This just started occuring last night, and now my computer only lasts anywhere from 20minutes to 3 hours before shutting down.

What could be causing this problem?

Thanks,
Adam J.

Your computer is running hot. Place an external table fan near its CPU air intake and let air flow in it through the fan and check if it reboots again.

There is a program called speed fan that checks your CPU temperature, fan rpms. http://www.almico.com/speedfan.php
Define shutdown. The windows bye-bye screen or is it like you pulled the plug?

Poblem with the power supply? Did you just fit a high end graphics card without adding a better power supply?

this problem gets much less frequent if you use a set speed in yor BIOS, sure you lose 2-300mhz, but its mucg more stable

possibilities:

1. too hot
2. inadequate power supply
3. bad/damaged RAM

yea PSU cloud be it, after i uprgaded my PSU to a 450w it significantly improved. but thene again if i just removed the 10 fans and car neon it might not use as much

Well everyones mentioned heats the problem, however:

If it's not the CPU or the Powerpack, then it's the graphics card. Sometimes they are known to hang the system and even crash them to reboot.

in any case the bet possible thing you can do if you feel up to it is unplug your computer, open up your case and use an old manual toothbrush to clean out the dust from the boards. You might want to unplug things like the RAM and even the cards and give a clean around them before carefully placing them all back in.

9 out of 10 times, just a clean and a refit of what you have is enough to deal with any small remittant problems, just hope that your time isn't that "1" time you don't want.

Update: I cleaned out as much dust as physically possible from my case, and I let a home fan blow on the air intake for awhile, then turned it on (with the fan still on)... 30 minutes later, it shut down again.

I also have an exclamation point next to Floppy Drive Controller in Device Manager, and on some of the Ports, too.

Also, when my computer shuts down, I have to press the button multiple times and sometimes even wait 10-15 minutes before it will start up again, only to crash soon thereafter.

Also, there is a "metallic whirring" i've been noticing from my fan, I don't know if this is connected, but just thought I'd say it.

How do I know if its the CPU / powerpack / graphics card? :/

does turning it off at the wall for 30seconds fix the proxblem of it not starting? if it does then im pointing at your PSU

but if its not. then try going into your bios and looknig ot the CPU temp. then go into your OS and run a high demand application until it crashes. and turn it on as soon as possible, then check the CPU temp in your BIOS again. if it is several(5 or more) degrees C hotter then that metallic grinding is probably your CPU fan not worknig properly

The ticking sound must mean that the fan or heatsink is obstructed or running at low speed, you will need a new heat sink and CPU fan, the case fan is probably working but the core temperature of the CPU is too hot, that will explain why using a desk fan would have no effect, a heatsink usualy costs around £15 from a computer shop