Please try to download the latest WUA installer from:
http://support.microsoft.com/kb/949104
now rename the file %windir%\system32\wuweb.dll to wuweb.dll.old.
In case we are unable to rename this file
Run this command line to find out which process is using wuweb.dll
Tasklist -m wuweb.dll -> dit laat het proces zien welk de dll vasthoud.
And kill the returned process.
Now you should be able to rename %windir%\system32\wuweb.dll to wuweb.dll.old.
Now install the WindowsUpdateAgent like descibed below:
with the following switch
WindowsUpdateAgent30-x86.exe /wuforce
After installation, please double confirm the version of %windir%\system32\wuweb.dll, make sure it is 7.4 or above.