I installed Conference XP 4.0 Beta on a Dell Dimension desktop 2 weeks ago and used a firewire camera without problems. Then last week, I completely reinstalled Windows XP with Service Pack2 and encountered a problem when trying to use 2 different firewire cameras. When I join a venue, I get a black video window for the video from the firewire camera. I get the same behaviour when trying to "test Video" in the Audio/Video Settings dialog box. I can receive video from other endpoints that have joined a venue. and my "black" video windows appears black on all other endpoints in the venue. This is reproduceable with at least 2 different firewire cameras.
The version of Conference XP installed is: CXP Client: 4.0.60926.0
My best guess is you need to install Windows Media Player 10 -- this is required by CXP 4.0 for video. Since you reinstalled XP Pro SP2, you probably only have version 9, which is the default. To get 10, your best bet is probably to run Windows Updates, click Custom updates, and choose Windows Media Player 10 from "Software, Optional" on the left hand side. Normally, I would just say go to the Windows Media Player website to get the software, but I did a quick glance over there and all I could find was version 11 beta 2. Unfortunately, 11 does not work with CXP yet.