One annoying thing that can happen to you on a Windows computer is your user profile can get corrupted in a way that prevents you from logging into your account. Instead you get the error message "The User Profile Service failed the logon. User profile cannot be loaded." Last weekend this happened to me for the second time on my Windows Vista machine. I am unsure of the cause, perhaps a power glitch at just the wrong time (I also got an error message about Windows not having shut down properly). Fortunately this time the fix proved relatively simple once I figured out what to do.
This is apparently a fairly common problem with lots of advice on the web about what to do. Unfortunately some of this advice seems flawed. For example this Microsoft page has several suggestions but they have drawbacks. For this reason once rebooting the system fails the next thing you should try is using the System Restore function to return your computer to an earlier working state. When this succeeds as it did in my case it solves the problem with little effort. However the Microsoft web page does not suggest this and instead offers some (in my view) inferior alternatives.
The first suggestion is to "fix" your user account profile by editing the registry. However as best I can tell this doesn't actually fix your profile, instead it replaces it with the default profile you get when you first set up your account. This will allow you to logon but you won't have your accustomed environment. I am not sure how serious the differences are as I didn't pursue this further.
The second suggestion is to create a new account and then copy most of your files from your old account to your new account. This is what I ended up having to do the first time I had this problem and it worked more or less. But it was quite a lot of trouble, things didn't end up exactly the same as before and I found the involuntary account name change annoying.
The earlier instance of this problem was a delayed effect of a malware infection. This was made worse by the fact that I (due to laziness) had not created separate user and administrator accounts. This is a bad idea which made fixing things harder as in order to create a new user account I first had to create an administrator account without being able to login normally which fortunately is possible but a bit complicated. This time my now separate administrator account was unaffected.
Be patient when using System Restore, I initially thought it was failing to launch but it was just taking its time (several minutes or so it seemed) to come up.