In Windows XP, KB936181, a Security Update for Microsoft XML Core Services 4.0 Service Pack 2 may repeatedly appears in the Windows Updates list even after being successfully installed. This could happen if one of the System files are corrupted in your Windows XP.
To resolve the problem open the C:\windows\system32 folder and rename the file msxml4.dll to msxml4.orig.
You can do the same from command line as follows:
1. click Start – Run and type “cmd”
2. Run the following command and ignore any error messages.
c:\> ren %windir%\System32\msxml4.dll msxml4.orig
Now, manually download and install the KB936181 from here to fix the problem.