Hi and welcome.
The problem could be many. Considering you built it withused parts it could be many things.
Is the motherboard and all the hardware compatible with Windows 7?
Did you check Microsofts compatibility checker program for Windows 7 before you installed Windows 7?
After you did a new install did you check and see if all the hardware drivers
were compatible with Windows 7?
I can go on and on guessing.
My solution is to install WinXP and see what happens. If it still crashes it is a hardware issue most probably.
Sorry I couldn't be any more help.
Good luck in trying to find the needle in the hay stack.