Originally pinched from acresso.com:

The following command line switches can be used with the uninstaller (IsUnist.exe or IsUn16.exe for InstallShield 5.x Professional, and Uninst.exe or Uninst16.exe for InstallShield3 and InstallShield Express).

Note: When passing a path and file name in a command line parameter, as you do with -f, -c, and -d, you must enclose a name containing one or more spaces in double quotation marks. If you don’t, the command line will misinterpret the command.

-y
Suppresses the message box that asks the user to confirm that uninstallation should proceed. The feedback dialog box is still displayed, as is the shared file dialog box (which is displayed when the reference count of a shared .dll is decremented to zero).

-x
Deletes all files, including those core components that normally are not removed. (All user interface elements are displayed.)

-f<log file name>
Specifies the location and name of the uninstallation log file. The syntax is as follows:
-f”C:\Program Files\Company Name\Deisl1.isu”.

-c<DLL file name>
Specifies the location and name of the external DLL that is to be used at the time of uninstallation. The syntax is as follows:
-c”C:\Program Files\Company Name\Custom.dll”.

InstallShield3 users can download the file UnDLL.exe (1,556 KB), which contains the InstallShield white paper “Calling a Custom DLL Function from unInstallShield” (Uninst.doc) and files for creating a complete InstallShield3 sample setup. Download UnDLL.exe to an empty folder. Execute it with the -d command line parameter in order to preserve the folder structure when you extract the files.

Note: This parameter cannot be easily used with InstallShield Express. This switch is already used by Express’s uninstaller and advanced modifications would have to be made to call another external DLL.

-a
When running in silent mode if unInstallShield encounters a shared file for which it would normally display the dialog box asking the user whether to remove the shared file, it will automatically reduce the reference count to zero and not remove the file. Therefore, running unInstallShield in silent mode is functionally equivalent to an uninstallation in which the user selects the “No to all” option when this dialog box first appears.

-d
Identifies a single file that is to be deleted. The display of user interface elements is the same as when the -a switch is used. The syntax is as follows:
-dC:\Temp\filename.ext.

(reference: kb.acresso.com/selfservice/viewContent.do?externalID=Q100021)