You didn't mention which version of SEP this is, and there was an issue with IU with older builds, resolved in MR4 MP1a.
Title: Intelligent Updater fails to update definitions when using the Version 5 updater.
http://www.symantec.com/docs/TECH92633
The log says:
Tue Oct 19 12:32:51 2010 : IU INFO: File-name : 20101018-002-v5i32.EXE
Tue Oct 19 12:32:51 2010 : IU INFO: Creation-date : 20101018
If it's a 64 bit machine, you'll need to get the 64-bit IU file (*v5i64.exe).
sandra