Okay, I've defined the problem and found solutions (could've sworn I tried every possibility last night). Apparently, it simply will not work with forward slashes (/) and a drive spec (C:).

It does however work with backslashes (\):

   'c:\path1\path2\file.exe'
[download]

or

"c:\\path1\\path2\\file.exe"
[download]

Also, case of the drive letter doesn't seem to matter. Thanks to everyone who responded. I would definitely call this a bug (at least with Win98 2ed/ASP 5.6.1).