1) Make sure the motherboard is on the latest BIOS.
2) Check that nothing is over-clocked. Ensure adequate cooling.
3) Reinstall Win7 from scratch. Do not apply any updates. Do not connect it to the network. Do not install any apps, games, or utilities. Keep it physically disconnected from all networks.
4) Test what happens when you "use" the machine that way for a few hours. Play some Solitaire, author some documents in Notepad.exe... if there's a crash, your hardware is defective almost beyond any doubt.
5) If you don't experience any crashes at all for several days, start very carefully and slowly applying updates and installing your apps/games. If the crashing resumes at a certain point, at least you'll know the likely trigger - it's the thing you installed last.