The Installation does roll back and it does come up in the events logs with the following message:
Product: Symantec Backup Exec for Windows Servers -- Internal Error 2704. .
And then follows it with this error:
Product: Symantec Backup Exec for Windows Servers - Update '{5A2190A6-38C0-4466-AE06-87B4C1CE34BC}' could not be installed. Error code 1603. Additional information is available in the log file C:\DOCUME~1\Backup\LOCALS~1\Temp\1\MSI8220.LOG.