Why does Spybot update service require secondary log on service?

On XP pro SP3, Pybot 2 beta.

As I only want to use Spybor as an on demand scanner, I have set all its services to manual and none are started. However, when I do a manual check for updates I got an error event in Windows event viewer when trying manual update, saying:
Event Type: Error
Event Source: Service Control Manager
Event Category: None
Event ID: 7001
Date: 14/06/2011
Time: 21:48:57
User: N/A
Computer: DJDRRM3J
Description:
The Spybot-S&D 2 Updating Service service depends on the Secondary Logon service which failed to start because of the following error:
The service cannot be started, either because it is disabled or because it has no enabled devices associated with it.

I then checked dependencies of the Sybot Updating service and it appears that it requires secondarey log on service to be started (or at least enabled).

I have always set the secondary logon service to disabled.

Why does Spybot S&D update service require secondary log on service?

No other software update that I have requires it.

My guess is that the updater uses an alternate user account.

I am not sure what you mean. This PC has only one user account, which has full admin rights (XP pro SP3).
The PC is set to auto log on to that account, and it is the one under which all software is installed (including Spybot, of course).