need help diag for pc

[tatertot]

my bro has a emachine im not sure which one but he is having a issue and i was wondering if any of you guys could send me in the right direction here is the symptoms and what i have tried

when the comp is fired up it will boot everything comes up correct but maybe 3 min into using it the screen goes black and the hd drive light stays steady ive tried a 2nd hard drive after i booted it the graphics were crap so i installed a pci video card graphics look good but it still did the same thing i changed memory same thing  i was looking at the internal parts and when this happens i notice the cpu fan stops running i can reboot as soon as it happens and all the same issue happens

so if ived tryed a hd ,memory .video card ,what should i look into next  possible processer?? mother board?? help  

i know emachines are not top notch but ive had good luck with 3 of them myself he cannot afford a new setup and im trying to help him out

thanks for any help in advance tatertot

[mipoikel]

Try new power next.

[Brooke]

I, too, suspect a bad power supply.

[eagl]

Could be cpu overheat if the heatsink isn't on correctly.  Try getting speedfan to see what temperatures it's reporting.  It could be a faulty mobo too.  Look for capacitors with bulged or burst tops.  If any are burst or have crusty stuff on them, the mobo is destroyed.

Also make sure the computer is running within specs.  I've seen machines that have been messed with, and when it flakes out the owner conveniently fails to mention that they moved the jumper from "200" to "266" or whatever...  Anyhow, make sure it's not being overclocked, FSB, cpu, or memory.

[tatertot]

ok i bought power supply same thing looking into the cpu fan next clean it out well but its my last resort then its a boat anchor probaly could have baoght him a new setup for couple hundred more but im trying to save him bucks he doesnt have alot ive checked all the easy issues if a 20 buck fan does it im mum lol he will kill me