DIFxAPI's DriverPackagePreinstall() returning ERROR_CANNOT_MAKE

My application is using the DIFxAPI DLLs to perform a driver install. The
program works in about a dozen different computers we have tested, with OSes
ranging from W2K to WIN7. We also have a 64-bit version (AMD) and a 32-bit
version.

But we have one Gateway tablet running WINXP Tablet PC 2005 which returns
ERROR_CANNOT_MAKE when either DriverPackagePreinstall() or
DriverPackageInstall() are called.

The DRVSTORE folder is present in C:\WINDOWS\SYSTEM32 (marked as
compressed). From explorer, I can create folders and copy files into
DRVSTORE. DRVSTORE contains a few drivers already, probably installed by
Gateway.

I'm using the latest DIFxAPI DLLs from WDK 7.1.0.

I really see no reason why DriverPackagePreinstall() fails. Does anyone
have any other ideas?