Well, I think I've finally solved the problem myself with this. I decided to try a graphics card from my other machine and see if it made any difference - this was a last resort decision as the HD2400 had been purchased as it was specifically recommended for being a capable HTPC graphics card. The card I replaced it with is an MSI Nvidia 7900GS card which is also passively cooled. Installed all the latest drivers (after of course uninstalling all the ATI stuff) and hey presto, no more interference issues in VMC! Fantastic!
I can only guess that either the HD2400 isn't powerful enough from a performance point of view, or that the 7900GS with its own power connector straight from the PSU gives it a more stable power supply. Your guess is as good as mine, but the HD2400 is basically being binned. What a load of rubbish!
Now I am having to get a smaller quiet cooling unit for the 7900GS card as the MSI one is far to big and prevents me from being able to put the lid back on the Zalman 135B case!
Kind Regards,
tricky_uk
Zalman HD135 : Asus A8R-MVP : AMD X2 4400 : 500GB Samsung Spinpoint : Samsung Writemaster SH-S203B : Nesteq Semi-Fanless ASM PSU (520W) : Hauppauge WinTV-NOVA-T 500 : MSI Geforce 7900GS with VF900: Vista Ultimate 32bit