Do you eventually come to a totally BLACK screen with a Cursor and the ability to hit ctrl-alt-del? If you can pull up task manager, it may be an Activation issue. If you can get into Task Manager, open a command prompt and run SLMGR.VBS /dlv and check the status of your activation.
This has happened to me with two legitimate activations, but I have also seen it happen when those "KMS Activators" are used, which are floating around the torrent sites. I am not encouraging the use of those.