I've installed Microsoft Security Essentials and my Windows XP Professional SP3 and it does not seem to install update signatures multiple times a day as I'm used to from other virus scanning tools. The signatures were two days old when I updated it manually.
My user account is a limited one, running surun if that might matter. But if I double click the MSSE tray icon and click the Update button myself, it updates, so I guess it can't be a permission problem.