I am posting this just in case it is helpful to someone else.  I think my problem is fixed (I will know for sure in a couple of days).

Environment:  Microsoft Windows Server 2003 64-bit running Exchange 2007, both with latest SP installed.  Symantec console on Exchange server showed that both the Premium Antispam and Virus Definitions protection components were correctly licensed and had about 20 days still to run (expiry December 2009).

I renewed my Symantec software licences by purchasing a new 12-month Multi-Tier Protection, and received the .SLF file from Symantec.  Copied the file to the Exchange Server, ran the Symantec Mail Security for Exchange console, went to Admin, Licensing, and installed the license file.

It threw an "unknown error":  80004005  

After the error, whilst the Premium Antispam component had updated (expiry December 2010), the Virus Definitions component was now showing as "Not Licenced".

After speaking with Symantec Support, we looked at the newly supplied .SLF licence file and noticed that for both the Preimum Antispam and Virus Definitions sections of the file showed a future start date of December 2009 (I was installing this in mid-November):

<key>
<name>SAVFMSE Virus Definitions</name>
<version>6.0</version>
<start_date>2009-12-09</start_date>
<end_date>2011-02-06</end_date>
<warn_policy>1,120</warn_policy>
<grace_policy>1,60</grace_policy>
</key>

<key>
<name>Brightmail AntiSpam Content</name>
<version>1.0</version>
<start_date>2009-11-12</start_date>
<end_date>2011-02-06</end_date>
<count>150</count>
<warn_policy>1,120</warn_policy>
<grace_policy>1,60</grace_policy>
</key>

On Support's recommendation, we tried installing our OLD .SLF licence file again.  Surprisingly, although it still threw an error message same as before, after doing this both the Premium Antispam and the Virus Definitions components showed as Licenced and also showed the correct expiry dates in December 2010.

I cannot explain why re-installing the old licence file fixed the problem, other than to speculate as follows:
-- installing the new file caused a problem because the start dates in the new file were later than the date of installation
-- reinstalling the old file fixed the problem because the start dates in the old file were earlier than the date of installation; ergo
-- maybe the licence file installation routine takes the earliest start date on all occasions, but only notices the end date if it is LATER than the previous end date recorded

Anyway, hope this helps someone.  Symantec Support also were able to provide me with instructions on how to manually remove the licence files, so if you ever get stuck you can try that and then reinstall your licences from scratch.