Microsoft Security Essentials Problems

I haven't heard much in the way of praise or criticism of Microsoft's security essential pack. It claims to provide "real-time protection for your home PC that guards against viruses, spyware, and other malicious software" [microsoft.com].

I decided to give it a go on my trusty XP laptop so, after giving AVG the boot, fired up the installer.

No installation problems, and the system scan ran fine and nothing to report, but over the time that it's been installed there's one odd issue that crops up at login:

Microsoft Security Essentials isn't monitoring your computer because the program's service stopped. You should restart it now.

Click 'Start now' to start the service.

The service always appears to start fine after I follow the instructions, but what's going on here? Is it a network issue that's causing it to give up?