it is not the easyest but motly managable. defrag your hdd (twice is my recomendation) make two partitions on the end of hdd (using qtpartedfrom knoppix or the commercial partition-magic...) , one about 500Mb (linux swap)and one about 4Gb ext3 (3Gb is about minimum)
then a /home as large as you think you can "afford" (mine is 2Gb but...), just remember that linux can read windows partitions but win canot read linux.

here is what seems to be a decent guide for installing (and much more).
http://godot.studentenweb.org/debian/index.cgi