I have no clue, but lately I've been having this problem. After running my workstation suddenly something destroys PATH and lots of stuff stops working. Initially, my path had MANY directories listed (I use Windows 7 as a development box) and after some time, I only had 3-4 directories listed in my path left.
I can't even open the System Properties dialog where I can see environment variables (because it uses shell variables to start it and it doesn't work). In short, I reboot every time.
Any ideas what might cause that problem? I didn't run anything new/unusual lately. Only VmWare (but I've been using it in the past and didn't have that problem back then).
I just checked, even
%WINDIR% is not defined. WTF?!