I've just had the same problem on a SQL2005 Standard Edition SP2 box that's not very active.

The only other thing that has logged an issue at a similar time on that box was McAfee, giving a McLogEvent EventID 257. (I've checked - McAfee On Access Scan is set to exclude SQL Server processes and mdf/ldf/ndf/bak/trn files.)

I've had a look into this machine's history, and I see that it suffered the same issue a year or so earlier. unfortunately, there's no DBA around here now who was here then that I can ask about it - and I've only been here 6 weeks!

The server itself seems perfectly happy, so I'm going to let it run.