Power supply weirdness: 300w vs 1000w in an old machine

Is there any reason why a 1000w psu shouldn't work in an old Athlon x64 system? It is backwards compatible with the 4 pin square 12v connection, and the computer boots fine with it's old 300w supply, but If I throw a 1000w into it, the PSU comes on, but the CPU fan goes full blast and it doesn't post.

Is the machine so old that it just can't handle that kind of power? You would think the PSU would scale down output as needed.

Thanks for any help.

Related thread on the death of my gaming pc:
http://hardforum.com/showthread.php?t=1800288

No, the issue is most likely something else.

The PSU will only output as much power as is being drawn from it, no more, no less.

Some older boards may require the -5v wire, which your 1000w most likely doesn't have. Check to see if there's a white wire on the main motherboard connector.

Or your 1000w power supply is defective.

Or it's one of those random times where a power supply and motherboard are just not compatible for unknown reasons.

Maybe the 1000w PSU is having some sort of problem.

On the 24 pin there is no white wire on either PSU.

What I don't get is why would the older computer power up the PSU just fine, but the new, previously working computer, won't even start it's fan spinning...