This problem seems quite common after installing SP3. I found these sugestions:
One solution might be:
Delete c:\program files\windowsupdate
Do the same to c:\winnt\system32\windowsupdate (a hidden directory)
Try windows update again.
The official way to clear out Windows Update is:
Some of these files are hidden files, please enable "Show Hidden Files and Folders" from your Folder options.
To delete Windows Update files:
Double click on My Computer, double click on C:\
Open the "WUTEMP" folder and delete the contents of the folder (If any files exist).
Double click the Program Files folder, then the 'Windows Update' folder. We will then need to delete all files & folders within this directory except the "V4" folder.
To do so, highlight the "V4" folder, then click Edit from the menu bar at top of the window and choose Invert Selection. If you cannot locate the "V4" folder, please skip this step, and then continue to delete the rest of the files by choosing Select All instead of Invert Selection.
Once all of the other files are highlighted, simply hit the Delete key on your keyboard to delete these files.
Next we need to delete all the files & folders within the V4 directory except the "iuhist.xml" file. To do so, perform the same steps as above by highlighting
the "iuhist.xml" file, then clicking Edit from the menu bar at top of the window and choose Invert Selection.
Once these other files are highlighted, again hit the Delete key on your keyboard to delete the files.
Next delete all the contents of the WUTemp folder (if it exists, if not skip this step).
Next Unregister old control:
Go to Start > run
Type: "regsvr32 /u C:\WINNT\System32\IUCTL.dll" (Without Quotes. You should get a unregistered successfully message)
Find your old site controls:
Go to Start > Search
Type "IUCTL.DLL" (Without Quotes)
Select the Search button.
Next Search and delete the IUEngine.dll file (Same as above).
Please delete any trace of the files IUCTL.DLL and IUENGINE.DLL
Reboot and try to access Windows Update again.
If that doesn't work, try the next one:
Empty the hidden directory c:\WUTemp. Try again. If that doesn't work...
Type the following URL in Internet Explorer
It will popup a dialog asking you to save iuctl.cab. Click on Save button.
Save iuctl.cab on your machine and extract its contents into a folder.
Once you extract you will have the following files:
Right click on iuctl.inf and select install. This will install the new iuctl.dll and iuengine.dll.
Open IE, clear out temporary internet files, and try again...
Hope this helps