I have exactly the same problem with one of my client's laptops (it is Dell XPS M1530 with Vista Ultimate). All the symptoms you described word by word. The timing of this problem's appearance is also similar (I assume you started to experience it about November/December 2011; the time when you wrote your post, please correct me if I am wrong). I was told about the problem first time in November (as far as I remember). I eventually remedied it by re-installing the system (I have replaced the hard drive as it had bad clusters, now I think it developed them as my client's only way to deal with frustration was to shut the laptop down again and again and again). However after about a month of smooth running the problem resurfaced in exactly the same way, shape and form. I suspect either a virus or a Microsoft update are causing this problem. Have you had any luck with sorting it out? If so, wouldn't you mind sharing your solution?
I have tried Malwarebytes (can't detect anything), Combofix (hangs forever when trying to generate report; eventually one must restart the machine and problem remains the same). There are some updates that the computer wants to have installed but I looked them up and they do not seem to be significant (Microsoft Office updates). Task manager as you pointed out never shows up and when I was trying to use Process Explorer it often hanged or did not work well.
The computer is also very slow with performing some very basic tasks and often hangs.
I have run full diagnostics but it seems that there are no problems with the hardware.
Edited by hamluis, 23 January 2012 - 04:32 PM.
PM sent new OP.