A VERY strange problem

hi, my comp specs are
processer: amd 1.8gig
vid card: nivida geforce fx 5200
512 sdram 40gig hard drive...running windows xp pro
and a 350watt powersupply, doesn't say on the power supply its self and not sure how i would find out but i am pretty sure its 350
now my problem is that my comp kinda shuts off at seemingly random times. what i mean by kinda shuts off is that the moniter goes off the sound goes off but the computer stays on...i just can't do anything and i have to manualy reboot my comp(press power butten)....I know it sounds like its in sleep mode and i am an idiot but trust me i know what sleep mode is and its not in sleep mode ...i shut off harddrive sleep mode in my bios and turned off the sleep mode in windows.....its like my comp shuts off but the power led is still on and i have done virus scans befor so i am pretty sure its not a virus...other then that i have no clue on what is going on

 

you need a better Power supply for your computer! try a 450 watts Psu!