My PC keeps on shutting down

Hey guys. I'm new here so don't be too critical. Thanks in advance.

So here's my problem - whenever I try to turn on my PC it shuts down when it gets to Windows loading screen (sometimes even sooner). I managed to check the BIOS, HDD is fine and working. Every single part seems to be good, according to my BIOS since it shows every single part that can be listed. A month ago I change CPU cooling fan, because the original Intel fan couldn't keep up with my CPUs (Specs in the end of the post) temperature (90C+). I've put a CoolerMaster Hyper TX3. Temperature fell down to 40C+ (never over 55C). Then a month later PC started shutting down once a day when being used, now it's impossible to turn it on. Maybe a year or two years ago, there was the same problem and the varranty guys changed my PSU from 400W to 430W and everything was ok. So i'm lead to believe that I have the same problem again. Is there anyway to check this without the multimeter (I don't have it)? I know how to check if it's GPU, going to do that tomorrow.


My PC specs:
Intel Quad Core Q6600 2.4ghz
PQI 2x2gb 1066mhz RAM
Gigabyte 256mb 8600GT (I know, I know... Old school)
Western Digital 400gb 7200RPM
MSI motherboard (don't know the full code/name of it)

I think these are all the things that matter.

Best regards,
Slimas