[PROBLEM]
On a 64bit Windows7, when trying to open a file with gVim in C:\windows\system32, it shows up empty, and vim says "new file".
Other files in the folder can be opened without problems.
It's not an access rights (permissions) issue, since the file can be opened with e.g. notepad.
[SOLUTION]
The inconsistent, confusing behavior that "some" applications see the file, while others don't, depends on whether the viewing application is 32bit or 64bit. gVim is 32bit, while notepad for example is 64bit.
The folder that actually is accessed from a 32bit application is in fact C:\Windows\systemWOW64.
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