Start by removing everything the system doesn't 'have' to have.
That's floppy drive, extra hard drives, extra optical drives, sound cards, graphics card (if the motherboard has an onboard one), any more than a single single stick of RAM.
Reseat everything, all power and data cables and all add in cards.
Reseat the processor, redo the thermal paste on it, check the CPU fan is connected, working, and pointing the right way.
Check the power supply fan is working.
Reset the BIOS to factory defaults.
Let it sit in the BIOS for 30 mins, check the temps every 5 mins or so and see if it's locking up.
If you have a spare optical drive, try the install using that.
If you have a different OS disk, try with that.
If you have a spare PSU, try that.
If you have different RAM, which is compatible with the motherboard, test it with that RAM.
If you have a different processor which is compatible with the motherboard, test it with that processor.
If you don't have the spares available, try running memtest off a boot disk.