How to diagnose why an update cannot be installed on Windows Server 2003?

On one specific server sometimes I get an error message when I'm trying to install security updates. The message is showed immediately after trying to install them. Where can I find the log files which show what's causing the problem?

Update: Here are some details from the WindowsUpdate.log, the first relevant line in the log is:

... Agent WARNING: LoadLibrary failed for srclient.dll with hr:8007007e