If the PS3 is ALREADY ON, and the PC is THEN switched on, the PS3 will not find Mezzmo UNLESS I logon to windows.

To be a little more specific:

1. If PC is already ON (whether logged on to windows or not), and THEN PS3 is switched on, Mezzmo appears immediately on PS3 without needing to use "Find Media Server"
2. If PS3 is already ON, and THEN PC is switched on (without windows logon), then "Find Media Server" does NOT find Mezzmo.
3. If PS3 is already ON, and THEN PC is switched on (WITH windows logon), then "Find Media Server" DOES find Mezzmo.

Obviously Mezzmo Media Server IS set to run on windows startup.

I can ALWAYS get the PS3 to find Mezzmo by stopping and restarting the Mezzmo Media Server.
Contrary to the advice on Conceiva, I MUST have IGMP Proxy AND IGMP Snooping ENABLED in order to get the PS3 to find Mezzmo, even in scenarios 1 and 3 above, though even here, a stop/start of the Mezzmo Media Server allows the PS3 to find Mezzmo.

It's not a complete show-stopper, but a nuisance nonetheless!

Does anyone have any ideas?

Mezzmo (latest version - 3.4.0.0)
Draytek Vigor 2850Vn (latest 3.6.4 firmware)
Windows XP
PS3 wired (switch then router)
PC wired (direct to router)