Sophos Enterprise Manager and SQL 2005

Hello Experts

Firstly please let me explain that although I am a network engineer of some years standing I have very little SQL server experience.

We have a server running Windows Server 2003 SP2 whose reason for existence is to run as our Sophos server providing AV updates to the network. All the PCs can conect to the server OK and pull down what they think are updates, but the problem I am having is that the server will not get the updates from Sophos.

The reason for this is the SQL Server 2005 (for Sophos) won't start. I say "for Sophos", but that is the only reason SQL is on there anyway. The event logs are not very forthcoming with assistance, but I have copied the "error" entries below for information:

Unable to cycle error log file from 'C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\LOG\ERRORLOG.4' to 'C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\LOG\ERRORLOG.5' due to OS error '5(error not found)'. A process outside of SQL Server may be preventing SQL Server from reading the files. As a result, errorlog entries may be lost and it may not be possible to view some SQL Server errorlogs. Make sure no other processes have locked the file with write-only access."

This one is repeated with different error log numbers, but the words are identical. Once that has repeated 6 times this is the next error:

UpdateUptimeRegKey: Operating system error 5(error not found) encountered.

The next one is:

Error 3(error not found) occurred while opening file 'C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\DATA\master.mdf' to obtain configuration information at startup. An invalid startup option might have caused the error. Verify your startup options, and correct or remove them if necessary.

And finally:

UpdateUptimeRegKey: Operating system error 5(error not found) encountered.

The server has been operational for over 2 years without problem until this cropped up at the end of last week.

As always, any pointers or assistance gratefully received.