Yesterday we had a strange issue on using the Dynamics NAV 4.02 client on a Vista x64 operating system. After applying the trick on sqlsrv32.dll, the error was this:
Surprise... the MDAC error is appeared again... seems that the dll was not hacked... 
After a bit of research, I've discovered that on Vista x64 there are two folders:
c:\windows\system32 contains sqlsrv32.dll (64 bit)
c:\windows\sysWOW64 contains sqlsrv32.dll (32 bit)
In order to run NAV 4.x on Vista x64 you've to modify the sqlsrv32.dll on the sysWOW64 folder by applying it the trick.
Remember that this is unsupported by Microsoft 
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Dynamics NAV