If you are running this system with a 300W power supply, it is time for an upgrade. I would recommend no less than a 450W. With that out of the way, also run some full hardware diagnostics. The Hirens Boot CD should cover all of that. If there are still problems after replacing the power supply and anything that fails hardware diagnostics, then update your drivers and codecs. If, after all of that, you still have issues, time for a clean install. Beyond that, you either missed something in the diagnostics or it's time to retire the system.
3Is there any possibility you are asking more from your power supply than it can supply? What your PSU capacity? Make and model? – irrational John – 2010-06-24T15:06:04.057
300 Watts. It really seems that it's PSU problem. Do you think it's enough? What power should I buy if not? – oneat – 2010-06-24T15:35:09.613
@oneat: look at the specs of each component, add up the power consumption, multiply by 1.5 (just to be sure) ... – akira – 2010-06-24T17:40:43.517
the problem is that I don't know power consumptions of each. – oneat – 2010-06-24T19:30:18.493
2300W is pretty low for a modern PC, particularly if the PSU is having other problems. – GalacticCowboy – 2010-06-24T19:32:59.257
@onehat: so, did you come to a conclusion? – akira – 2011-01-25T05:18:33.697