Could not load file or assembly 'stole, Version=7.0.3300.0'

We have an application which has a C# exe which calls a VB 6.0 dll through an interop.
Problem is that on some vista machines it gives following error while crating interop.
Could not load file or assembly 'stdole, Version=7.0.3300.0'
I solved this problem by putting the same file in application path folder.
But my question is that why stdole.dll is not on that machine.
From where does it normally install, and why on certain pcs I am not finding the same?

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stdole.dll is a Primary Interop assembly (PIA) for office.
check the link:

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