Re: so what is wrong?
i would try another PSU, even if it works fine with your other system.
PSU lose efficieny after awhile of use, so it doesnt give as much energy as it did when you first got it. So im assuming that the PSU doesnt generate enough power.
other than that, im assuming its the motherboard.
RAM would give you post beeps, CPU will not post when turned on, graphics card will also give you beeps.
Desktop -Cooler Master NV690/AMD Phenom II X4 965 BE/MSI 880G-E45 Motherboard/2 x 2GB DDR3 G.Skill Ripjaws X/Sapphire HD 5870 1GB GDDR5/CORSAIR CMPSU-550VX/Seagate 7200.12 500GB (OS), Samsung F3 1TB (Storage)/Sony DVD Burner/Windows 7 Professional 64-bit