WinNT, WInXP, Win2000, Win2003 - user-names are NOT case sensitive. Fred=FRED=fred.
Passwords ARE case-sensitive.
(From Microsoft Documentation - compare this with filenames in Windows: polly.exe=Polly.exe=POLLY.EXE)
Linux, Unix (and all variants) - usernames ARE case sensitive . Fred, FRED and fred are 3 different users.
(This is true even if your favourite distro allows uppercase or not)
Your problems stems from the fact that you have allowed your users to dictate your security policy.
As I see it, you have Windows user Fred trying to access an Ibay as user "Fred" when the Ibay expects "fred".
There is a Samba fix for mapping user Fred to be user fred - sorry I don't know what it is, but I remember someone having to do this where I used to work in the dim mists of time.
If it is just a matter of users connecting to Ibays, get them to make a permanent connection, using a different username. (See "Tools/Map Network Drive" in My Computer - there is a link to "connect using a diffrent username". Make sure the "Reconnect at logon" is ticked).
Good luck