Power pack fuse blows when pc turned on

[myselfme]

hi
im new here was hoping someone could shed some light on my problem,
a friend asked me to fix her pc saying that when she put it on one day it blew a fuse and wouldnt come on ,

so i took the pc and my husband had a look and changed the power supply from an old pc i had here and it worked,
so i formatted hard drive installed software and was ready to go back today , but when i put it on to put some more software on , it blew i had a fright as it sparked kinda like when a fuse blows on a plug ,
what could be doing this ?
any help much appreciated
thank you

 

[hunter315]

What are the specs of the system? What power supplies have been in it? It might be shorting something out and causing PSU's to explode.

 

[myselfme]

amd sepron processor 3000 +
IDE : Nvidia Nforce 3 250 pararell
2gb ram

Its got all the original hardware it aint been upgraded ,
the power supplys original one that was in there was :

"Atx 400watt switch n power supply" hubby said (inside the fuse had gone)

so he put on this one and it worked until i put it on this morning :

"Model ATX 500 Watt"

 

[popatim]

I suspect the capacitors on the motherboard have weakened. Have him check them. They look like little soup can sitting on the motherboard and just like soupcans they should not have the tops domed up on them nor should the appear to have leaked (brownish gunk at the bottoms or sides) WWW.badcaps.net has some pictures of motherboards on the bottom right side of the webpage if you'd like to know what to look for.