PC freezes sometimes and sometimes restarts

For sometime ,my PC is bugging me out  
When i do something ,that doesn't necessarily require a lot of memory ,like when i hit the back button on my browser and want to open a page at the same time ,but also in games ,my PC freezes .When the freezing occurs not ingame ,all programs stop responding and whatever i do ,it wont respond again and after some time of clicking ,my cursor freezes too and i'm left to manually reset .
When it freezes in a game ,the sound repeats itself over and over again and same thing :reset .
All i've said happens randomly .I mean sometimes it freezes ,sometimes it doesn't .
I've had the game freeze last summer too ,with an older GPU and i'm afraid that my current GPU will end the same .
And now to the sudden restarts ...
I think this is really random ,because it hasn't happened to me while playing a game ,yet .When I browse the web or watch a clip (on the web) ,the PC restarts .But sometimes it happens on the desktop too ,after ive manually restarted (because of the first problem i mentioned ) ,only sometimes ,it doesnt happen every time i do that .
It might be the PSU failing ,or the CPU ,or even the GPU in games or the heat ,right ?But how do I test that ?
My system specs are :
GPU nVidia PX9600GT Winfast Leadtek
CPU AMD Athlon x64 3500+ 2.2GHZ
PSU FEEL 450 ATX
MO-BO nFORCE4M-A
A 232 GB WDC HDD
2GB of RAM
I posted a pic with my temps

http://shareimage.ro/viewer.php?file=g1fde5u4m4h0woz58l...
Please ,if you have an opinion or advice about this ,do post .It might be useful.