The problem is probably unrelated to the printer, unless there used to be a fax facility liste with the printer.
ISDN refers to a dial-up modem. Does your friend have one fitted? More important does he use it?
Two things you can try-
Start > Run > (and type) msconfig
Click on the Startup tab and look for a modem related file with a tick in the box on the left. removing a tick will stop a program from loading when you first boot up.
Go into Control Panel and click on System then on Device Manager.
You should see a list of hardware, none of which should have a yellow triangle with a black exclamation mark on it.
If you make any progress, post back here with what you have found/done, and hopefully others will steam in to help you.