I realize many of you may have given up and just reinstalled Vista or moved on to W7 at this point.
I was having the same issues as Coherent Computer Solutions. I had narrowed the issue(s) down similarly and had gotten to the point where it seemed likely to me that the WMI issues were somehow related to the inability to install SP2. I too was getting "Can’t Collect Information. Cannot access the Windows Management Instrumentation software. Windows Management files may be moved or missing." Attempting to install Vista SP2 consistently wound up with "No such interface supported" Details - "Error: E_NOINTERFACE(0x80004002)" I had tried all the other suggestions found throughout this thread, especially in the sixth post.
I had already run the command "net localgroup Administrators LocalService /add" to resolve some problems with network connectivity. I was receiving the message "Connection status: unknown. Server execution failed" when attempting to launch the Network and Sharing Center. The "net localgroup Adminsitrators....." command fixed this beautifully.
But I was still getting the WMI error(s) and, as mentioned, also attributed the inability to install SP2 (No such interface supported) as being probably related to this. More digging and wmidiag showing similar errors reinforced this belief. Finally, a bit more searching turned up this command:
net localgroup administrators NetworkService /ADD
Make sure to run it from an elevated command prompt (right-click and run as Administrator). But, after running this command, msinfo32 worked perfectly and, lo and behold, Vista SP2 installed cleanly (and rather quickly, actually). I hope this helps someone else out there and thanks to Coherent Computer Solutions for the msinfo32 WMI lead. Happy holidays to all.