Declare Sub ... Lib "...\my.dll" stopped working after xp sp2

Hi!

I really hope that someone could help me.

I have a vb.net application that uses the unmanaged dll, which is part
of an independent software. The code is:
Declare Sub SUB_NAME Lib "c:\path\file.dll" (ByRef lngStatus As
Integer)

Previously I could succefully call the sub and everything worked ok.
However after the installation of XP service pack 2 I started to get an
exception: System.DllNotFoundException - Unable to load DLL
(c:\path\file.dll).

I believe this is some security issue, but how do I fix it?

Thank you for any advice.

Grigory.