Can't update Windows on the Windows Update Website.
Automatic Update for Windows Doesn't work.
Windows Defender cant download updates.
Windows Defender update not available and displaying an error code.

This problem work around is for Windows XP and Windows Vista.

First
Check that you have Administrative Rights and that your connection to the internet is working.
To do this Go to your "Control Panel", and left click on "User Accounts"
Make sure your profile has your Account Type set to Administrator.

A quick way to check if your internet connection is working, try your Web Browser.
If you can surf the web, your connection is working.
If  you cant surf the web, and you could before, you need to resolve your internet connection issues.

If you internet connection is good and you are an Administrator and Windows Update, and /or Windows Defender update still does not work, try this:
Re-Start your computer, and upon boot up start pressing the F8 Key.
This should bring up a menu offering boot up options.
Use the cursor keys to highlight the "Safe Mode" option, and press the "Enter" Key.
After some time you should reach windows desktop in "Safe Mode".
It will look quite different, this is normal, do not be alarmed.
A help screen may appear, if it does, close it.
Click "Start" go to the program menu, and go to accessories.
In accessories go to "Windows Explorer".
In Windows Explorer left click next to "Computer" to expand.
Left Click Drive C
Left Click "Windows"
Scroll to the "SoftwareDistribution" folder and right click on it, then left click "rename"
Press the right cursor key to position your cursor at the end of the name, and type old at the end  and press enter it should now be named "SoftwareDistributionold"
At this point re-start your computer normally.
Your ability to update should now function.

If you are not satisfied you can reverse the modification in "Safe Mode" by simply deleting "SoftwareDistribution" and rename the folder "SoftwareDistributionold", removing old from the end of the folders name so that it is once again "SoftwareDistribution". Then re-boot.

Sometimes the definitions you already have are corrupted, preventing automatic update - this fixes that
To manually install the update definitions, follow these steps:
Download the Latest Windows Defender Definitions
Run the downloaded file to install the latest definitions.

Install this Utility
Select all the Windows Defender entries listed in this utility, and then Click "Remove".
Reboot your Computer
Reinstall Windows Defender

Download Dial-A-Fix Here.
Extract Dial-A-Fix with IZArc: <--Click this link to download IZArc or use another .zip archive extraction utility of your choice.
Run the "Dial-a-fix.exe" file by double left clicking on it.
Click the box "Fix Windows Installer"
Click the box "Fix Windows Update"
Click "GO"
Reboot your Computer

If that didn't do it:
Run Dial-a-fix.exe
Click "Flush Software Distribution""
Select "Yes"
Reboot your Computer

If that didn't do it:
Run Dial-a-fix.exe
Click "Flush Software Distribution""
Select "No"
Reboot your Computer

Open Command Prompt
(Located in Accessories on your Program Menu (After Clicking Start) or Click Start>>Run>>Type cmd in the box>>click OK)
Execute the Following Commands one at a time followed by pressing the "Enter" key and clicking "OK" to close the pop ups:

regsvr32 wuapi.dll
regsvr32 wuaueng.dll
regsvr32 wuaueng1.dll
regsvr32 wucltui.dll
regsvr32 wups.dll
regsvr32 wuweb.dll
exit

Reboot your Computer