I recently had a DSL speed upgrade and suddenly my PMS would not connect my PC to either of my 2 PS3's. The PS3 icon would not show up in the PMS GUI after a restart server, and even though the WMP icon appeared, neither PS3 could see any shared folders (either from PMS or using WMp as a media server). Before this network change everything connected beautifully and I was streaming media from a central PC to both PS3's all the time without any problems.
So for the past 3 evenings I tried everything on the router, everything on the PS3, and tons of stuff on Windows. Everything was powered up and down and re-booted dozens of times. I read hundreds of threads from people having similar issues.
Reading here, I came across these threads about IGMP Proxy and it seemed it should be disabled, and it was by default. So i did not change that setting for 3 days. But after exhausting all other possibilities, I said WTF and enabled it under the "Advanced/MulitCast" area of my Frontier Netgear 7550 router. Immediately, PMS found the PS3 on the PC interface, and when I went to both PS3's, all shared folders were there!!!
I do not know WTF this IGMP Proxy and Multicast are exactly, even if I read some network mumbo-jumbo about it while researching this problem. But I know changing it worked, so don't be afraid to try.