Sometimes your computer turns on as you'd expect, you get to the Windows login screen, but then something happens. Your computer might freeze up, reboot on its own, or just stop and not respond to anything you do.
Maybe you see the login screen but after entering your password, nothing happens. On the other hand, maybe you can log in but then Windows freezes and you have to reboot manually. Then again maybe Windows seems to start but your desktop never shows up and all you can do is move your mouse around a blank screen.
Regardless of the specifics, this is the troubleshooting guide to use if Windows starts most of the way but you can't get logged in or your desktop never fully loads.
Important: If you don't even get to the Windows login screen, or you see any kind of error message, see How To Fix a Computer That Won't Turn On for some better troubleshooting steps for your specific problem.
Time Required: Anywhere from minutes to hours depending on why Windows is stopping during the login process
A failed update or one-time startup process can sometimes cause stopping, freezing, or reboot-loop issues during the login process. Often times all Windows needs is a clean boot into Safe Mode and then a restart to clear up the problem.
Start Windows with the Last Known Good Configuration. Starting Windows with the Last Known Good Configuration will return driver and registry settings to the state they were in the last time Windows started up and shut down properly, possibly returning your computer to working order.
Of course this will only work if the cause of your Windows login issue is a registry or driver configuration issue.
Note: It is safe to try Safe Mode before Last Known Good Configuration because the valuable information that's stored in the registry to make Last Known Good Configuration work properly isn't written until Windows starts successfully in Normal Mode.
Repair your Windows installation. A common reason for Windows to fail between the login screen and the successful loading of the desktop is because one or more important Windows files are damaged or missing. Repairing Windows replaces these important files without removing or changing anything else on your computer.
Note: In Windows 7 and Vista, this is called a Startup Repair. In Windows XP it's referred to as a Repair Installation.
Important: The Windows XP Repair Installation is more complicated and has more drawbacks than the Startup Repair available in later Windows operating systems. If you're using Windows XP, you may want to wait until you've tried Steps 4, 5, and 6 before giving this a try.
Start Windows in Safe Mode and then use System Restore to undo recent changes. Windows could freeze, stop, or reboot during the login process because of damage to a driver, important file, or part of the registry. A System Restore will return all of those things to a time when your computer was working, which could solve your problem entirely.
Note: If you can't enter Safe Mode for some reason, you can also perform a System Restore from System Recovery Options which is available from the Advanced Boot Options menu in Windows 7 and also from your Windows 7 or Windows Vista Setup DVD.
Important: You will not be able to undo a System Restore if it's done from Safe Mode or from System Recovery Options. You might not care since you can't get to Windows normally anyway, but it's something I wanted you to be aware of.
Scan your computer for viruses, again from Safe Mode. If you're having problems even getting that far, see my list of Free Bootable Antivirus Tools for some programs that will scan for viruses even without access to Windows.
A virus or other kind of malware might have caused a specific enough problem with a part of Windows to cause it to fail during login.
Clear the CMOS. Clearing the BIOS memory on your motherboard will return the BIOS settings to their factory default levels. A BIOS misconfiguration could be the reason that Windows can't get all the way to the desktop.
Important: If clearing the CMOS does fix your Windows login problem, make sure any changes you make in BIOS are completed one at a time so if the problem returns, you'll know which change caused the problem.
Replace the CMOS battery if your computer is more than three years old or if it's been off for an extended amount of time.
CMOS batteries are very inexpensive and one that is no longer keeping a charge can cause all sorts of strange behavior at any point during a computer's startup process, all the way up to the loading of the Windows desktop.
Reseat everything in your computer that you can. Reseating will reestablish the various connections inside your computer and could clear up the issue that's preventing Windows from fully starting.
Try reseating the following hardware and then see if Windows will fully start:
Check for causes of electrical shorts inside your computer. An electrical short is sometimes the cause of problems during the Windows login process, especially reboot loops and hard freezes.
If your system memory is failing, your computer may freeze, stop, or reboot any point, including during or after the Windows login process.
Replace the memory in your computer if the memory test shows any kind of problem.
Important: Make sure you've tried your best to complete the troubleshooting steps up to this one. Steps 11 and 12 both involve more difficult and destructive solutions to Windows not starting fully. It may be that one of the below solutions is necessary to fix your problem but if you haven't been diligent in your troubleshooting up to this point, you can't know for sure that one of the easier solutions above isn't the right one.
Test the hard drive. A physical problem with your hard drive is certainly a reason why Windows might not start fully. A hard drive that can't read and write information properly can't load the files necessary for Windows to start.
If no hard drive issues are found then the hard drive is physically fine, meaning the cause of your problem must be with Windows, in which case the next step will solve the problem.
Perform a Clean Install of Windows. This type of installation will completely erase the drive Windows is installed on and then install the operating system again from scratch.
Important: In Step 3, I advised that you try to solve this issue by repairing Windows. Since that method of fixing important Windows files is non-destructive, make sure that you've tried that before the completely destructive, last-resort clean install in this step.