As you are asking for that and it eventually boots you can read what it takes to dig into a cranky windows with articles like http://blogs.technet.com/b/markrussinovich/archive/2012/07/02/3506849.aspx
I can't do more than guide for a few reasons. First I'm not there to watch, hear and there is no single cause or cure for this one. But since it boots you can on a good boot get the tools to watch a bad boot to log what's going on and possibly find the root cause.
Most folk don't want to find root causes but instead reinstall the OS to its factory condition and if that fails it's always been a hardware failure.
This began just a couple days ago. When I press the start button, laptop makes all the right noises, the Lenovo logo appears, then the screen goes dark--not black, it doesn't turn off, it looks like the blue Windows desktop is about to come up, it just doesn't. After numerous attempts, it will finally boot, but there doesn't seem to be a rhyme or reason as to why. I've made a boot disk, but would like to get to the root cause.
Any thoughts or suggestions? Thanks in advance to anyone who can help.