BTW, thanks for the help (before I forget to say it later!)
I've been trying to fix this all day, and during that time have made sure that all drivers installed are the latest I can find. The only drivers I couldn't upgrade were for the NIC, as it seems to have been discontinued. I found some XP drivers for it but couldn't for the life of me convince Vista to actually install them.
Flow control - the drivers that come with Vista don't have an option for flow control. Annoyingly, I know the XP ones do but, as I say, can't convince Vista to install them.
50% cpu - just made the problem worse - that time took 7 minutes (well, a few seconds under) to start playing the video to the program.
Assuming, and from what you're saying it sounds most likely, this is a NIC problem, I'll try plugging in an old USB1.1 ethernet adapter I've got - the performance isn't really good enough for MCE but it might help test whether that is where the problem lies.
In the meantime, any more suggestions you have are most welcome!