I never knew this until today, but the Windows filesystem *can* be case-sensitive.
The HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\kernel\ dword:ObCaseInsensitive registry key controls filesystem case sensitivity. If set to 0, the filesystem is case sensitive, if set to 1, it is case insensitive.
Note: After you install a Microsoft .NET Framework 2.0 update, a file system’s case sensitivity changes from case sensitive to case insensitive. (See Microsoft KB Article 929110)