PC Shuts down by itself

20secs after logging in, my pc shuts down by itself..I have to unplug it before it will turn on again but eventually if will turn off again after few secs... is it a PSU or Motherboard failure? I check my Mobo and PSU and there was no blown capacitors...but I can smell a a burnt capacitor..help me pls..=]

Most likely overheating. Hope that its not too late
Test temperture.
Cupid is good software for the test.

hard to say without seeing the machine in person.
If you smell something burnt inside, you have to figure out if it's the PSU or mobo or both.

Also, when you say "20 seconds after logging on it shuts down", are you saying that it will boot to Windows and then shut down?
Because you then state that after you unplug it and turn it back on it turns off after a few seconds.

If you have a multimeter handy or can borrow one, Britton30 has a great http://www.sevenforums.com/hardware-devices/201055-psu-testing.html#post1688197 thread. You can click on the image to enlarge it, or save the full image and view bigger on your own machine.

.bat file causing a shutdown?