I have this problem too but the suggested fix won't work for me. AutoCAD 2012 still runs fine but the AutoCAD 2011 which I downloaded today will not launch, in the same way as described by users in this thread.
I cannot check the ADLM file or rename the FLEXnet file because in my PC (XP, 32 bit) the file tree is not as described in your fix.
Your fix says C:\Documents and Settings\All Users\Application Data\Autodesk\ADLM
In the 'run' box, I try to open C:\Documents and Settings\All Users\Application Data\Autodesk\ADLM but I get
"Windows cannot find 'C:\ Documents'. Please make sure you typed correctly... "
However, I can get as far as the application data folder if I cut the last two items off
C:\Documents and Settings\All Users\Application Data\
but that application data folder does not contain a folder or file called Autodesk or AutoCAD.
I had expected to be prompted to verify my license the first time I launched but there was no such prompt.