Dell 4600 power supply replacement problem

I recently had to replace my power supply in my Dell 4600 Dimension desktop computer. Went to best buy and they put in a Dynex power supply model
DX-400WPS Now my computer is running very very slow. I have run register cure over 20 times but did not help.
Any ideas why this is happening?
I called Dynex and they say it is a configuration problem with Dell and so I called Dell and they say that is not true, there is no settings or anything that needs to be changed. But they won't troubleshoot it because it is not a Dell replacement part.
Very frustrated here.
Any help or suggestions would be greatly appreciated.

DeltaDaylate

Just look at this video and see how that PSU explodes

Yeah, I have until the 8th of June to return the Dynex. I am thinking of going with an Antec, but my question now is, what wattage? The old Dell one was a 250 watt and they put in a 400 watt.

Thanks so much for your help.

Yes thats the one, zipzoomflyhigh listed. For 39.99 after MiR its a really good deal.