dell dimension e520

I have dell dimension e520. It was working fine then sudenly I got blue screen. took it to Fry's for repair and paid $70.00 but after a week they told me the mother board is damaged . I bought a new motherboard but it did not solve the problem. I thought may be the processor is damaged and bought a new E600 Dual core 2.8 GHZ 2m shared L2 Cache 1066 FSB LGA 775 65 W Dual- Core(SLGU9)Processor . I read about this new processor and every article said it is compatible with the original one.After installing evrything I turned on the computer but nothing hapened.only black screen. no boot procecess or dell start up. just black screen. please tell me what the problem might be. I use window XP professional . I love this desktop and want to repair it. please help I do appreciat any input.
thanks
Ray