Vista - computer shutting itself down

soo im not sure if this is the place to be posting this because im not sure if the problem is a security one. but anyhow, my computer keeps shutting itself down randomly. sometimes its 5 minutes, sometimes its 20. The screen just fades out to black then the computer shuts down. Any ideas as to why? also my internet isnt working on that computer, though all my connections seem to be fine. These both happened this morning when i turned on my pc. I'm really at a total loss for what to do.

over heating sounds like the culprit, check the fans on your cpu and the video card. look on your main board there are to large processors they may have either heat sinks only or have heat sink and fan. if not turn the machine off pull the plug from back of computer and put finger close to the chip a see if you feel excessive heat come off the chips, another words to hot to touch. follow anti statics rules. what you do is this touch case to discharge any static built up on your body.

what are you doing when it shuts down?

also if it is up for only 5 min and shuts down doubt it is heat releated.

does it just shut down or does vista shutdown correctly?

The PSU could be on its last legs. You could try exchanging it for another.

As other's mentioned, more then likely overheating or bad power supply are causing the computer to shut down suddenly. Your network issue is of more interest to me. Are you running wireless or wired? If you're running wired or are running wireless and are able to connect to your router, open up a cmd prompt (win r type in cmd or type cmd in start menu and click cmd.exe) and type in ipconfig. If the first octet in your IP address is 169, then you're not talking to the DHCP server. If you have a router your IP address first two Octets should be 192.168, or if you directly connect to your modem it should begin with anything but 127 or 169.