MSDN: If the access causes the system to display the UAC prompt, redirection does not occur. Instead, the 64-bit version of the requested file is launched. To prevent this problem, either specify the SysWOW64 directory to avoid redirection and ensure access to the 32-bit version of the file, or run the 32-bit application with administrator privileges so the UAC prompt is not displayed.

Try SYSNATIVE instead?