Access Violation using CreateParameter

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I've just started trying to update a small utility that hasn't been
touched in about six months and found that it now crashes whenever
CreateParameter is called, simple queries work fine but as soon as
anything with variables of any kind is attempted I get the crash. The
specifics are:

"Unhandled exception at 0x4bc0145c in MigrationUtil.exe: 0xC0000005:
Access violation writing location 0x00000000."

Breaking into the code at that point I find that the actual error is
occurring in msado15.dll, but I have no debug information for that
point, the last point in the code I control is at the CreateParameter
call. All the values passed in seem correct and it hasn't been changed
since the last time it worked. I can't find any bad pointers anywhere
so I'm wondering if it might be some kind of problem with msado
itself. Unfortunately in the interval we've switched from VS7.1 to
VS8.0, switched from x86 to x64 build machines, and in the process
upgraded our version of MDAC, so I'm not sure what might be causing
the problem.

So far the closest thing I've been able to find to this problem is
this one: http://support.microsoft.com/kb/911570

However the hotfix as described applies to version 2.82.2589.0 of the
msado files and I'm currently at version 2.82.3959.0. Based on the
description I did try switching the strings we were using from wchar_t
to char, but strangely all that did was cause the access violation
address to change from 0x00000000 to 0xCCCCCCCC.

Here's the call stack as it currently appears:

msado15.dll!4bc0145c()
[Frames below may be incorrect and/or missing, no symbols loaded for
msado15.dll]
oleaut32.dll!00653ff5()
msado15.dll!4bc5003c()
ADO.dll!Command15::CreateParameter(_bstr_t Name={...}, DataTypeEnum
Type=adVarChar, ParameterDirectionEnum Direction=adParamInput, __int64
Size=4000, const _variant_t & Value={...}) Line 1095 + 0x63 bytes C++

At one point yesterday debug_alloc showed up in there somewhere, but
I'm not getting that now for some reason.

Can anyone suggest what might be the problem, or at least where to
start looking?

Thanks!

.