I am hoping that someone can help me with a very troubling Boot-up problem that has been occurring for the last few weeks.

When I start up my computer, the initial boot sequence PROCEEDS normally up to the point at which the pale blue  "Welcome" screen appears.

Up until this problem started, the Desktop would load a few seconds after the 'Welcome" screen appeared now however the "Welcome" screen persists for at least 30 seconds, the Vista ring-throbber eventually appears, and then the whole screen goes completely black with the exception that my cursor shows and can be moved with the mouse.

The black screen persists for anything up to several minutes until eventually the Vista ring-throbber re-appears followed by the Task bar which in turn is followed by the whole Desktop a few seconds later.

On two occasions there has been a fleeting error message that appeared in the black screen indicating that " some logon failed"  with SOMETHING, but the screen is so transient that it is gone before I can really make a note of its exact wording.

I should add that once the desktop has loaded, the computer works completely normally. It has not slowed down at all  functions are as they should be.

One other thing that I THINK may be related is the FACT that I am unable to create a new user account on this computer.
To be more specific, I can go through the motions of creating another user account using 'User accounts' in control panel, and when I have created the new account it does appear in the "manage accounts" dialogue box with its icon as it should. But when I restart the computer, the boot sequence is completely unchanged to that which I have described above, specifically I am never given an opportunity to log onto the new user account.
And, more to the point, when I look at my folder tree in Windows explorer, under "Users" the only "Users" that appear are: Default; Public; and my Name.  the user account that I have just created is not listed and naturally has no files ASSIGNED to it.

I am sorry to have been so long-winded but I have been attempting to describe the problem accurately, and I do hope that someone can explain what is happening and more to the point, offer a fix. 
Thank you.  Is it legit version? Will it start quickly in Safe Mode?