I have had this problem before in VW 11.5.1 I figured out that something in my custom workspace had become corrupt. I had to delete the custom workspace from the workspace folder so that VW would not try to load it. VW then loaded just fine; although , I had to rebuild my workspace. In your case, since workspaces are specific to the version VW11.5 would still work while VW12 would not.
This was also on a WinXp machine.
Hope this helps.