If you know how long this has been going on and do a system restore to where the PC was working fine, you try that. Otherwise, you create a new profile. You can copy over the documents from the old to the new and just delete the old profile and the contents of its folders.
When I started Windows Vista recently, on the screen where I select the user, I clicked on the usual one and got "User Profile Service failed the logon. User Profile cannot be loaded." There was an OK button, which logged out and returned me to the user selection screen. All other users worked fine. My files are still there and accessible.
How can I fix this?
As the admin, when I looked at the manage users screen, all users were there and appeared normal.