How can I tell which version of MDAC I'm running?

The simplest way is to use the version property of the ADODB.Connection object, such as: 
 

<%      set conn = CreateObject("ADODB.Connection")      response.write conn.version      set conn = nothing  %>

 
Another way is to go to Start, Run... and type regedit, then look at the following key: 
 

HKEY_LOCAL_MACHINE      \Software          \Microsoft              \DataAccess                  \Version

 
You can read this registry key from T-SQL, provided you are connected as a user with sufficient rights: 
 

EXEC master..xp_regread      N'HKEY_LOCAL_MACHINE',      N'Software\Microsoft\DataAccess',      N'Version'

 
Yet another way is to find msado15.dll (in %SYSTEM32%\DLLCache\), right-click it and hit properties, and look on the version tab. This should produce the exact same result as the registry scan above. 
 
You can also use the component checker, available from download.microsoft.com. Information about using this tool is available in KB article KB #307255. 
 
For some other ideas, see KB #301202. 
 
For a history of MDAC releases, see KB #231943. 
 
To obtain the most recent version of MDAC, see MDAC Download Page.