Let me know which fix worked for you

A. Try another profile with Administrator access.
If you do not have one, go to User Accounts in the Control Panel.
Select "Create a new account"
Select "Computer Administrator" when the option arrives.
Follow the prompts to create the profile.
Log out of old profile, log in to new profile.

B. If you have Norton Internet Security:
Disable Parental Controls

Error: "You must be running a Microsoft Windows operating system in order to use Windows Update"

C.  Wininet retries POST requests with a blank header

E.  Clear Internet Cache
1. Click Tools > Internet Options
2. In the "Temporary Internet Files" section, click "Delete Cookies"
3. Click "Delete Files" click "Delete all offline content" click OK
4. In the "History" section, click "Clear History".
5. Click "OK".

F.  System Date and Time
1. Click on Start, Settings, Control Panel (or Start, Control Panel)
2. Open the Date/Time item.
3. Verify that you have the correct date and time on your computer.
4. Verify the correct Time Zone.

I.  Check the language setting in Internet Explorer:
1. Open Internet Explorer, click Tools > Internet Options
2. Click on the Languages button
3. Verify at least one language is listed

J.  Check the host file for IP addresses associated with Windows Update.
If you are running Win98/ME, the HOSTS file will be in this folder:
C:\Windows
If you are running WinNT/2K/XP, it will be in this folder:
C:\WINDOWS\system32\drivers\etc
Double click the Hosts file to open in Notepad
If you find IP addresses associated with Windows Update please remove the entries and save changes.

K.  Please uncheck "Check for server certificate revocation":
1. Open Internet Explorer
2. Click on Tools
3. Click on Internet Option from the drop down menu
4. Click on the Advanced tab
5. Scroll down to the Security Section
6. Unselect "Check for server certificate revocation"
7. Select "Do not save encrypted pages to disk"
8. Click OK, follow prompts and reboot the machine

L.  Since SSL cannot be cached many users can access the site using the secured link directly:
https://v4.windowsupdate.microsoft.com/en/default.asp

M.  Deleted

N. Repair Internet Explorer
Windows XP:
How do I repair Internet Explorer in Windows® XP?

Windows 98/ME:
Start/Settings/Control Panel
Double click Add/Remove Programs
Double click "Microsoft Internet Explorer..."
Select "Repair Internet explorer"
Click OK and follow prompts.
Reboot.
If you do not have the latest version of Internet explorer, update now.
Download Internet Explorer to your desktop.
Reboot
Disable all unnecessary applications including antivirus
Install Internet Explorer by double clicking the downloaded icon.
Follow prompts and reboot.

O.  Windows Update requires ports 80 and 443
Verify your firewall and or proxy has both ports 80 and 443 enabled for both incoming and outgoing traffic.

P.  HOW TO: Troubleshoot Situations Where You Cannot Complete MSN Sign-up or Connect to SSL Secured (128-Bit) Web Sites by Using Internet Explorer in Windows XP

Q.  Check the following certificates are installed and valid:
1. Internet Explorer >Tools > Internet Options > Content >Certificates
2. Select the Trusted Root Certification Authorities tab
3. Check for a certificate called "MICROSOFT ROOT AUTHORITY".
4. Double-click on it to open its properties.
5. On the General tab, verify valid dates "1/10/1997 to 12/31/2020"
6. On the Certification Path tab, Certificate Status, make sure it says "This certificate is OK." Click OK,
7. Find a certificate "NO LIABILITY ACCEPTED".
8. On the General tab, verify valid dates "5/11/1997 to 1/7/2004" Although this certificate is expired the site will continue to work.  The site will fail if the certificate is missing or revoked.
9. On the Certification Path tab, Certificate Status, it may say "This certificate is expired or not yet valid." Click OK. Click the Close button.
10. Find a certificate called "GTE CYBER TRUST ROOT". You may have more than one of these with the same name. You need to check the one that has an Expiration date of 2/23/2006.
11. On the General tab, make sure that the Valid from dates are correct. It should be "2/23/1996 to 2/23/2006"
12. On the Certification Path tab, under the Certificate Status section, make sure it says "This certificate is OK."
13. Click OK on the certificate. Click the Close button on the Certificates screen.

R.  Register the following system files
Click Start > Run
Type “regsvr32 softpub.dll” (w/o quotes)
Press OK
Repeat the above steps for the following:
regsvr32 wintrust.dll
regsvr32 initpki.dll
regsvr32 dssenh.dll
regsvr32 rsaenh.dll
regsvr32 gpkcsp.dll
regsvr32 sccbase.dll
regsvr32 slbcsp.dll
regsvr32 cryptdlg.dll

S.  If TLS 1.0 and FIPS are both enabled, users will receive the 0x800c0008 error message on the first GetManifest
They must disable FIPS.
System Cryptography: Use FIPS compliant Algorithms policy
1. In Control Panel, click "Administrative Tools", and then double-click "Local Security Policy".
2. In "Local Security Settings", expand "Local Policies", and then click "Security Options".
3. Under "Policy" in the right pane, double-click "System cryptography: Use FIPS compliant algorithms for encryption, hashing, and signing", and then click "Disabled".The change takes effect after the local security policy is re-applied.
For more information:
PRB: Cannot Visit SSL Sites after You Enable FIPS Compliant Cryptography

T.  Try it in Safe Mode With Networking (Windows XP)
Reboot tapping F8 each second.
Select "Safe Mode With Networking" at the menu.