Hi A A Gruntpuddock, CTRL+ALT DEL doesn't work at all when the system freezes, nor when the Logging Off screen is "stuck".
I work with computers (as a controls engineer, so by necessity rather than being an IT genius) and all the usual stuff to try to diagnose the fault either isn't accessible while the problem is happening or shows nothing helpful afterwards.
Forgot to mention I'm running Kaspersky Internet 2011 as my all round antivirus etc program, with which I've had no problems since it was installed earlier on this year.
I've also done the following (out of desparation rather than anything else):
1) Carefully cleaned out the inside of the PC in case the issue was related to overheating. There was a little bit of fluff, but nothing to write home about. The fans are all working OK, and the heatsinks were all fluff free. I've seen much worse!
2) Uninstalled IE9 (I'm now using IE8). Prior to uninstalling IE9 a Windows error message had appeared saying that IE9 had stopped working (just before one of the crashes). An "update" was automatically downloaded for IE9 after Windows had "searched for a solution". So I'll see what happens when I log off now I'm back to IE8.
Fingers crossed.
EDIT: during the uninstall of IE9 the PC needed to restart. The dreaded logging off screen appeared, so I just left it...and left it...and left it. The log off successfully happened after 25mins, then the PC restarted OK. So it looks like the log offs would have actually happened eventually if I had left it long enough....so the problem seems to be exceptionally long log off times, rather than the log off not happening at all and requiring a power cycle.