Scratch CNC Mill

Hi all, I'm glad I was able to find this gem of a forum on the WWW.
Anyways, I'm building from scratch my own mini cnc vertical mill.
Everything from the screws (1/4" all thread rod) to the software (my
high school C++ training :). I'm accessing the ports directly using
_outp, but Windows seems to be interupting the flow of data every 3
seconds, upon which the 2 phase steppers (from old printers) give a
buzzing noise for half a second or so. Anyone know what this could
be and how to fix it? That, and what is the standard way to
calculate the path of the cutter on an arc? I was thinking using the
standard equation form of a circle, incrementing x by one pulse, and
then moving y according to the result. Of course, special care would
have to be taken when the maximum and minimum range values are
reached.