XP wont start- hangs on black screen.

Running Dell Dimension 3000 PC. XP home.
No recent additions to PC.

Upon startup today the PC loads briefly with DEll bar across btm of screen.
Then the screen goes black- and remains black.
I have tried to start from CD Rom- by changing order of startup in BIOS.
No luck- screen says >press any key to boot from CD.
>Setup is inspecting your computers hardware configuration.

After that the screen remains black.

I notice that when I hold down on/ off button on front of Pc the PC immediately switches off- rather than take few seconds.
All 4 diodes are green on the back of the PC