Right, are you ready for this? I am tempting fate by even saying this, but my problems now are SOLVED!!!! Here's what happened:-
I can't count the number of things I have tried, but last night, I finally gave up and decided to format and install Vista 32bit instead. Rebooted with the Vista CD in and went through the installation process. Didn't appear to be any problems until it was just about to login for the first time, and the screen just froze with the desktop wallpaper. No icons, no taskbar, no clock...JUST the wallpaper.
I couldn't believe it. Never had problems like this with Vista. So, I had to do a hard reset and try again. Same deal. It just froze with the wallpaper. No disk activity. Nothing.
I then started to think about what had fundamentally changed since trying to upgrade to Windows 7 from Vista. The significant thing was a new hard disk: a 1TB Western Digital WD10EADS Caviar Green model freshly purchased. I hadn't had any issues with it: in fact, I'd even had Windows 7 run a chkdsk on it at one point and it found no problems. But, I decided to disconnect it and instead use one of my original 500GB drives (Samsung Spinpoint) as the boot drive instead. In fact, for simplicity, I disconnected all my hard disks bar this one and went through the Windows 7 installation process (again) to this drive, formatting along the way.
The install seemed to go quicker (but that could have just been in my head). Everything seems to be working just fine. I have downloaded all updates, and installed all drivers and everything is working like clockwork. Tried to force a crash by going in Media Centre, starting a program, stopping it, minimizing it, opening internet explorer etc etc but it seems rock solid.
I then brought my second 500GB Samsung Spinpoint back online by reconnecting it to the motherboard and importing the disk using Win7 Disk Management. This disk has all my photos, music, recorded TV etc on it. It is still working perfectly!
I haven't tried to reconnect the 1TB drive yet: I will give it a few days and see how things go.
I am not 100% about partitions and things like that, but I recall in my first attempts with Windows 7 that the 1TB (boot) disk was set to a primary partition. One of my 500GB drives was also set to a primary partition. The other 500GB drive was set to a Dynamic disk. These are three physical drives. I don't know if having 2 primary partitions in there was the problem, or just that Windows 7 doesn't like the model of WD drive that I was using. Now, my boot disk is a primary partition, and the second disk is still set to a Dynamic disk. I don't know whay pone of them is Dynamic, but I know that if I tryu to change it to a 'basic' disk in Windows 7 Disk Management, it tells me that all data would be lost. So I have just left it as it is.
I will try re-connecting the 1TB disk at some point soon as I really could do with the extra storage space.