No Boot Dimension 8400..?

I'm new to Dell Comunity.

I'm hoping the community can help me sort out a problem with my Dimension 8400.

When I power-up I hear continuous beeps; nothing on the display, not even the bios prompt.  This has happened previously and seems to happen when I turn the power off at the wall socket after closing down - If I leave the power on at the wall socket all seems OK; I.E. the PC will boot if left in 'stand-by' mode.

On previous occasions when I turn the power on at the wall socket I hear the cooling fan start up; sometimes the PC will boot even though I have not pressed the power-up button on the front panel, if it does not boot-up I have tried booting reperatedly by pressing the power-up button on the front panel; after quite a few attempts the PC booted; but, not this time - Doh..!

Any help greatly appreciated...

I have noticed a little while ago that the cooling fan makes a kinda screeming noise; is this a connected symptom or separate..?