InstallScript function IS(FILE_EXISTS) returns false

[sks2004]

The InstallScript function IS(FILE_EXISTS) recently started failing for me on all Vista target machines, if the installation is running from a network. Running from a CD works.
The code I'm using is:
if (Is(FILE_EXISTS, svIniSource) = TRUE) then

It recently started returning FALSE when running the installation from a network, and always works when running either locally or from a CD-Rom.
This is a Vista issue only. This functions as expected on non-Vista machines.

I have repro'd this on numerous domain and non-domain, Vista machines and virtual images (W23 servers hosting the files and WV64 Business workstations hosting the files)

[MichaelU]

If by network drive you mean a mapped network drive, it's possible that there's a context switch getting in the way. UNC paths should generally not be subject to this, unless the administrative account lacks access that the non-administrative account has. To verify if this is the cause of the problem, try showing the path you are about to check, then check manually if it exists in both a normal and an elevated command prompt.

[sks2004]

Good test, I'll give it a try.

I am referring to UNC. Bypassing the test for file existance, I can also replicate this via XCopyFile().