OK, this problem came back a couple of days ago for no apparent reason but this time I think I've fixed it properly!
The 360 connects to the Vista PC via a windows login that is automatically created on that PC. I have fixed the problem by disabling the screen saver for this user but I had to change some registry settings to do it. Only proceed if you know what you are doing!
1.) Turn on the 360 and load up the Media Center Extender frontend.
2.) Bring up a command prompt window on the PC and run the regedit utility.
3.) Browse to the registry location:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\hivelist
4.) Make note of the registry user key associated to the Media Center user (probably Mcx1 although I assume there maybe an Mcx2 etc. if you have more that one extender). See this
screenshot.
5.) Browse to the registry location:
HKEY_USERS\[registry user key from step 4]\Control Panel\Desktop
6.) Change the ScreenSaveActive key to 0 by double clicking it. See this
screenshot.
Vista Home Premium RTM (32 bit), AMD 64 3000+, MSI RS480M2, ATI Radeon X1300 256MB, 1GB DDR400, 200GB Samsung SpinPoint HDD, Samsung WriteMaster DVD±RW, Hauppauge Nova-T 500