I'm not a PS3 user so I don't know how much I can help. Maybe someone else can chime in.
If you can access the internet from the PS3 and see the files on your PC, it seems some ports are being blocked. AFAIK the firewall in the 2wire modem only blocks internet ports so that shouldn't be the problem with sharing files from your PC.
If you or MTS installed ZoneAlarm, try temporarily shutting it down by right-clicking on the icon and selecting Shutdown. If that doesn't solve it, check your Windows firewall to see if it is blocking any ports.
From the reading I did I'm guessing you're using Windows Media Player on the PC to share the movies. Double-check both the PC and PS3 to make sure the IP addresses are assigned dynamically (not static). Also double check that the file sharing and other settings in WMP are correct on the PC.