When I boot I can read the bios but when windows starts the monitor goes blank. I tryed to replace the hard drive, but when I try to boot the cd, the monitor is blank. I have replace the vidio card but same thing is happening. The Bios detects the memory and cpu; there are no beeps. What's wrong?

Does your motherboard have onboard video? Kind of sounds like your BIOS is set to the wrong VGA output. Post your specs if you can, please, it'll help.

Or, try plugging that monitor (or a different one) into the onboard video port (if it exists).

There are points during the boot process that the display will go blank. On some systems, this can last anywhere from a few seconds to a few minutes. It depends on a number of factors.

Also, while the display is blank, over a period of at least a minute or two, see if the hard drive activity LED continues to flicker/light up. If it doesn't, your system may be "freezing" during boot.

Next time it happens, wait at least 5 minutes before you declare that nothing is happening.

By the way, shutting the system off while it continues to boot (even if you can't see it) is one way to corrupt the filesystem.