Using AutoCAD 2011 we always use 0.00 as our default line weight but the default will change to 0.010 with no real pattern. It can be any drawing at any time. We have reset this dozens of times without being able to stop it from reoccurring.
We are also not able to get line weight to display (yes we check the Lineweight Settings, Display lineweight box) again on random drawings at random times.
maybe some lisp/application that does that?
go to other machine with neat installation.
if you do not have another machine you can logon to your machine with a new user name
this will create a neat installaion.
remove any acad.lsp, acaddoc.lsp & *.mnl files from the support file search path.