Unidrv-based bitmap driver sample in windows DDK can not work

[Daeaeshew]

To learn how to program a virtual printer driver, I compile a sample
(Unidrv-based bitmap driver) in windows DDK.
The compilation is according to guidance from DDK user guide and everything
is OK.
However, after I add the compiled Unidrv-based bitmap driver to system and
try to print a test page,
it fails to print the test page.
An error message, which indicates the Unidrv-based bitmap printer fails in
printing, emerges at the right bottom corner of the desktop.

In the research, I found if we changed the port type of the printer from
"LPT1" to "COM1", then the problem disappeared and the printer could print
successfully.
You may change the port type in the "Control Panel -> Printer & Fax ->
Bitmap Printer (right-mouse-button click) -> Property -> Port".
I guess maybe this problem is caused by ports conflicting.

In addition, on another machine, which is XP SP2, the sample bitmap driver
works OK.
Could you please tell me why this happens? My Windows OS version is XP, SP3.
Thanks.