Win32Exception error code 267 "The directory name is invalid"

I'm currently working on an "Automated Run As" tool. Its goal is helping admins which, like me, have to give users a means to execute one or two programs as Administrator and would like to do so without having to surrender an admin's password.

So, I'm developing on Vista and I just whipped up a small proof of concept prototype, that'd run calc.exe as a different user, using ProcessStartInfo and Process. This worked fine when I executed it as myself (a rather pointless exercise, I must admit), but when I created a new user and tried to run it as him, I stumbled upon a Win32Exception complaining that the directory name is invalid, native error code 267. I was instsantly baffled, as I knew of no supplied directory name that could be invalid. I then tested the code on an XP machine and it worked!

I started googling on it to no avail, many reports of that error but no conclusive solution, or on different contexts. Finally, after a while it dawned on me, I wasn't specifying the WorkingDirectory property of the ProcessStartInfo class, as soon as I added the lines

FileInfo fileInfo = new FileInfo(path);
startInfo.WorkingDirectory = fileInfo.DirectoryName;

to my code, it was allowed to run code as different than logged in user.

As to why, I haven't looked deep enough yet, I think that it may have something to do with default values of the property or with default behavior when it is not specified, plus default permissions (which were tightened on Vista, if I'm not mistaken).

Do *you*, my non existent reader, know why?