POST is power on self test. Windows cannot load until this has been done successfully, therefore it is in fact posting.
From what you have said, I think a faulty video card or monitor, perhaps a bad connection somewhere is causing the problem. Thats what I would check for first anyway.
Ok, I have to ask this: Are you sure its not just taking a long time for the monitor to warm up, like how a TV doesn't display immediately after turned on, but takes a little while?