One day instead of connecting to the database my SQL Server Management gave me the following error: "Data Source name not found & no default driver found"
Note it had been working for several years before this, all that had changed was an upgrade to Office (97 ->2000) and the monthly Windows patches.
I tried rebooting, re-installing both the SQL Server 2000 and 2005 clients on the machine but no joy. I can access the database(s) from other machines.
Just to save anyone else who may have had this (it took me a full day to work it out), it turned out to be a registry problem:
Several keys were missing from the following registry path, I exported the key from another machine and it's all working now.
HKEY_LOCAL_MACHINE\SOFTWARE\ODBC\ODBCINST.INI