My interest was sparked by my Grandfather, and the stories he told. Numerous visits to the Viking Ship museum in Oslo as well as other medieval sites and museums also did its part. In addition I have to admit that a general interest in history has always been an important part of my life.
The reason I chose the two first voyages, i.e. North America and Constantinople, was that these two destinations are not frequently thought of in relation to the Viking raids. As such I had to do a fair amount of research to make the trips "correctly" described, and the research part is my favorite part while writing the books.