Error message: "User Profile Service service failed the logon. User profile cannot be loaded.''

This is the dreaded error message you might get for reasons unknown to you.  Maybe you've been hit with malware or somehow deleted your profile without knowing it.  The former is more likely than the latter.  Until you fix it you will not have access to your profile.  I've seen this error ALOT so I would classify it as common.

It’s always a good idea to have a second account on your computer with Administrator rights for situations like this. And you should be running with your main account as a Standard user for security reasons.  If you have a second account you can quickly log into it and fix this problem or access the files in your corrupted profile.  If you don’t have a separate account, your screwed!  ;)  Just kidding. Actually, there is a great post on this at the link below. Follow steps 2 and 3 on the “Answer” post right below the author’s first post. http://answers.microsoft.com/en-us/windows/forum/windows_vista-security/user-profile-service-service-failed-the-logon-user/4ed66b21-c23e-42f1-98b2-706dcf931fae

Ok, so hopefully you’re now in your newly created account or the one you had already.  Open a RUN box by pressing Windows key+R at the same time.  Type regedit.  This will open the Windows Registry.

Navigate to: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList

You’ll see several folders under the ProfileList. You can find which folder belongs to which profile by clicking on it and looking at “ProfileImagePath” on the right.  You'll see the path and profile name out to the right.

**Please read this entire post before making any changes so you fully understand how to fix this and so you don't epic fail your regedit.

 What your looking for is the folder linked to your profile starting with "S-1-5" with an extension of ".bak". 

Now, right click this listing and choose "rename" then take the ".bak" extension off the end.

You might also have 2 folders with the exact same string of numbers and only one has the ".bak" extension and the other has no extension.  If this is the case you need to take a few steps to rename them properly. 

First, rename the string WITHOUT the ".bak", adding ".ba" at the end. 

 Then, rename the string WITH ".bak" and remove the ".bak" extension.

Finally, rename the string with ".ba" to ".bak".  Your entries should look similar to this now.

Exit the regeditor and restart your computer.  When it reboots you should now be able to log into your main account. If this does not solve your problem then the corrupted profile is probably unrecoverable. You then want to go into your extra account and retrieve all of your data and copy it to a newly created Standard user account.  Then you can delete the old profile.