The error is "Additions to this Web site have been blocked." It used to happen occasionally in WSS 2, and now it's confirmed to happen in WSS 3. What am I talking about? MOSS installations where no one monitors the size of database growth.
Bring up SQL Server Management Studio and check out where the errors are being thrown. It's usually the transaction logs. If you're using log shipping or have a regular database backup strategy (you mean you don't?) the logs will maintain a reasonable size since the operations regularly trim the backlog. If not, then the transaction logs will grow, and grow, and grow. Since you're not doing anything with them, you're just as well to truncate the logs on a routine basis. For SQL 2005, I like comment 14 in this post, it does a great job of describing a way to shrink the logs in the UI, and how to eliminate the log file completely.
Best practices? Have separate spindles for your SQL Server OS, databases (.mdf files), and transaction logs. Monitor the growth of your data and transaction logs, and follow a regular backup strategy.