Audio, but no video from non-HD TV channels on client machine

[oberon000]

TV-Server Version:
MediaPortal Version:
MediaPortal Skin:
Windows Version: Vista Home Premium 64-bit
CPU Type: Core2Duo 6850
HDD: 1TB Samsung
Memory: 4GB OCZ (2x2GB)
Motherboard: Gigabyte GA-P35-DS3R
Video Card: Inno3D NVIDIA 8600GT
Video Card Driver: 7.15.11.6925
Sound Card: On-board Realtek HD Audio
Sound Card AC3:
Sound Card Driver: 6.0.1.5449
1. TV Card: Compro T300
1. TV Card Type: DVB-T
1. TV Card Driver:
2. TV Card:
2. TV Card Type:
2. TV Card Driver:
3. TV Card:
3. TV Card Type:
3. TV Card Driver:
4. TV Card:
4. TV Card Type:
4. TV Card Driver:
MPEG2 Video Codec: MPV Decoder filter
MPEG2 Audio Codec: MPA Decoder filter
h.264 Video Codec: ffdshow Video decoder
Satelite/CableTV Provider: -
HTPC Case: -
Cooling:
Power Supply:
Remote:
TV:
TV - HTPC Connection:


Hi. Have successfully installed and use MP to watch TV on my HTPC, and it works great. Have just installed it on my desktop, and am watching TV as a client (server is the HTPC).
MP works perfectly on the HTPC (running XP). Above stats are for my desktop system.
MP installs without a problem on my desktop, sets up OK, but when I try to watch TV, I get an issue.
High Definition channels are fine, I recieve both audio and video. When I try to watch a non-HD channel, I recieve the audio, but all I get is a black screen for the video.
I am a new user of MP, still looking to find my feet, so any help would be appreciated.

Thanks in advance
oberon