How can I change current printer....

Hi, All

I would like to change current printer.
Whenever I change the PrinterIndex of TPrinter object,
I always meet a error message.

            "Printer selected is not valid."

My printers are 'HP-600K', 'HP-660' and Text Only Printer.
And all printers are connected with network line.

If you have any comments about this problem,
Please let me know.