My computer doesnt not work properly?

there are several problems with my PC
- there is nothing displayed on my Monitor
- my PC shuts down in a couple of seconds after i turn it on .
Update: there are several problems with my PC
- there is nothing displayed on my Monitor
- my PC shuts down in a couple of seconds after i turn it on .( now it does not)
-i have changed the graphics card, still does not work
-i have changed the PSU, still doesnt work.
-=/
Update 2: -i have an asus p5s800 - VM mobo
-2gb of ddr ram
-pentium 4 3.0ghz
- 9250 ati card
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