Any PC hardware experts here, please?

[Zac67]

If the machine seems to start for a split second and then shuts off, it's usually an electrical short.

Have you verified that the CPU's orientation is correct and no pins are bent?
Checked seating of AGP card? The two staggered rows of contact are an ample source of trouble.
Replaced thermal paste? If you re-used the old grease with a Socket A CPU (Athlon/XP/Duron), you might have killed the chip. Or you might have cracked the die when replacing the cooler, sometimes that shorts the power.

PS: FSC 3.4 GHz probably means a P4 - are the pins in the socket (LGA775) or on the CPU (Socket 478)? The LGA sockets are very easy to damage...
But it's next to impossible to remove an LGA775 CPU by accident.  :-?

[Zac67]

kd7ota wrote:
I could see that the LGA775 is next to impossible, but with Socket 478 its possible because ive done this 3 different times with other people's computers.

Yes, very easy actually. You should try twisting the heat sink a bit before pulling to soften the thermal pad. Works most of the time.

@OP
If that happened to you:
- clean CPU / heat sink from thermal pad/paste residue
- check pins - they must be 100% straight
- open the socket by lifting the lever
- carefully plug the CPU to the socket, note orientation
- hold CPU in place & lock with lever
- re-apply thermal paste, sparingly
- mount heat sink, make sure to orient it correctly
- don't forget the fan power