Someone from Microsoft helped me out with this a few years ago (back in the days of Windows XP) and gave me a solution that met my needs at the time. I already had two drives in my machine, and what I needed to test was already on the D drive, and fortunately I didn't need to keep any files open on my D drive when I wanted to flush the cache. From the Windows Disk Management interface, I could change my D: drive letter to another letter, and then back again, and the D drive would perform as if had a cold cache in the OS.
