Computer won’t boot

I have a friend with a problem that needs a quick answer and a quick fix. The problem is this, the computer will not start up if both the modem and printer is plugged into their correct places. We have already checked the PCI cards and ports are plugged into the mainboard correctly and they are all fine. We have tried restarting the computer a number of times, and only sometimes it will actually start with both modem and printer plugged in, so therefore the problem seems to be caused by the printer. We do know that the printer is correctly installed, and no error or conflict messages appear. This same printer has been successfully installed with another computer and all works as it should. Can anyone figure out this problem effectivelyand quickly for us?