Upgraded CPU, doesn't run?! Plz Help

[mbarb15]

After getting tired of my old computer running like crap, i decided to rebuild my computer. The rebuild included just about everything. New motherboard, processor, ram, video card, and eventually a new poweer supply.
Processor: AMD Sempron 3100 754 chipset.
Motherboard: MSI K8M Neo-V 754 chipset
RAM: 512 DDR PC3200
Video Card: Gforce Nvidia FX 5700 optima AGP
PSU: 500 watt 115v-230v dual fan Comp USA brand

Tho, after putting everything together (minus the new PSU unit, at the moment) using my old 300 watt PSU unit, i would see both the PSU fan and the processor fan spin, but no power would be provided to the rest of the hardware (cd-rw, floppy, hard drive). Someone told me that the reason why the power isn't reaching the hard drive, etc.. is because the PSU is not providing enough wattage to run them. So i decided to replace it with a bigger PSU (500 watt). After installing the new unit i plugged the computer on and hoped for luck. When i hit the power switch both the motherboard fan and the PSU fan spun for a second and then shut off. (Let me also included that no image was ever signaled to the monitor). Thereof, i rechecked all of the wiring, played with it for a little and still could not decipher the problem.
If anyone knows what it could possibly be plz help, i would gladly appreciate it...
Thanx,
MikeB

[elroy]

You'll love this-
Remove the board from the case and put it on a non-conductive surface. Install the cpu, cpu fan/heatsink, one stick of ram and a video card.
Connect the power supply and power up by shorting the 2 pins together where you hook the power switch. Just short together for about 2 seconds. It should start. Often times something is shorted inside the case or the board and case aren't fitted just perfect and the AGP video card is not completely seated. The AGP card must be firmly seated in the slot.

[crossedup]

Make sure the HSF is seated securely as well, if its tipped a little and not making full contact with the CPU it can cause it to overheat very fast and do the same thing.

Go with Elroy first though, sounds like a shorting issue.