Please clean the temporary folders (usually C:\Documents and
Settings\username\local settings\temp and c:\Documents and
Settings\username\local settings\Temporary Internet Files\Content.IE5 and
delete all the files in those directories and subdirectories). Then try to
perform the update again.
If it does not help, there is a manual update procedure. Follow these steps:
Right click on MSAS icon within systray and choose shutdown.