If you don't really know the type of what the variant holds (in your example, it seems to be a string represented as a VT_BSTR), the best and safest way is to call the Windows API VariantChangeType (or VariantChangeTypeEx is localization is an issue); here is an example (not boost-specific):

VARIANT vIn;
VariantInit(&vIn);
vIn.vt = VT_BSTR;
vIn.bstrVal = ::SysAllocString(L"12345678");

VARIANT vOut;
VariantInit(&vOut);

// convert the input variant into a 32-bit integer
// this works also for other compatible types, not only BSTR
if (S_OK == VariantChangeType(&vOut, &vIn, 0, VT_I4))
{
    // now, you can safely use the intVal member
    printf("out int: %i\n", vOut.intVal);
}    

VariantClear(&vOut);
VariantClear(&vIn);