Event Viewer Warning: MSFTPSVC event 100

Archived from groups: microsoft.public.windowsxp.general (More info?)

On checking my Event Log this morning I found 274 entries (yes I am sad
enough to count them!) with the following message:
The server was unable to logon the Windows NT account <a/c> due to the
following error:

Where <a/c> were a variety of account names that I do not have on this
machine. It had all the apperance of something trying to get into the
machine and guessing at account names.

The events occurred between 03:10 and 03:11 BST.

This is the first time I have observed such messages. One possible item of
relevance was that a few hours earlier I installed the new MS automated
update service, and set 03:00 as the time for checking updates.

Anyone else observed something similar? anyone with any thoughts on the
matter?