Canon BJC-4100 like a matrix printer now

Any suggestion how to make my BJC-4100 print solid characters and pictures again?

Problem: it seems to print only every second dot vertically. It makes graphics and text look faded and bad. Exactly like this guy had 3 years ago:
http://www.freetechsupport.com/_disc8/000001ab.htm


The printer has gone through a new computer and conversion from win98 to winXP now without any problems and has been working just fine for a couple of months. Last week the problem started????

Thanx for any suggestion.