I actually have PMS running on Windows 7 Beta, build 7000 (x64) right now. It worked right out of the gate for me. I only had to tell it where my media files were, and there was a tutorial that noted boot up sequences for first time discovery that was important.
1. Install PMS, reboot computer.
2. With PMS TURNED OFF, go to your PS3 and ask it to scan for media servers. It should *not* find your PMS server.
3. Then go to your PC, and start PMS.
4. Back to the PS3, then have it scan for media servers again
5. Back on the PC, PMS should now see the PS3.
That should get ya going. If you start PMS first time, while your PS3 is on, they won't see each other on any subsequent boot up until you go through the above power on/scan sequence. (I'm hoping I remembered it right - I saw it on another site I can't find now.... ).