Most likely the graphics adaptor.
Is your monitor CRT? If so it's possible that the Card has been set to a refresh rate or resolution that the monitor can't handle: Try starting in safe mode. (Should say, though, that if this is the case, the monitor would work during the post test and initial load of Windows, and fail when loading your settings.)
try checking your RAM sticks. i had the same prob a few months back. i check everything and had to be the thing i least expected to be wrong. LOL. If not the ram then i dont know. sorry cant be more help
Chances are there's a message on your desktop telling you your resolution is set too low or similar but of course you can't see it. I had this and found the message using Logmein remote desktop. Turned out I had to replace the graphics card.
Thanks guys, dont know if i have explained this fully, let me try again, my monitor doesn't even turn on so i dont see the boot up sequence or anything. The monitor does work because I have plugged it into my laptop and it went ok. The other thing I should mention is that there are no longer any beeps when i turn the thing on. Any ideas or suggestions would be welcome.
Well it could be the graphics adaptor, but equally, if the machine doesn't boot, you would get the same symptoms - i.e. no video.
If you are not even getting a beep code from the POST test, then that could be a failed power supply or a failed processor, of a duff mother board.
In other words, it could be practically anything!