There are a couple options:
One - when you restart you can hold the 'Option' key down and then choose Boot Camp - however this is a temporary selection so if you restart again it will start back up in OS X (or whatever the selected startup disk actually is).
Two - You'll need to find out the disk ID by running
df -k - more than likely it is /dev/disk1s3 if you have partitioned your hard drive but you'll want to double check. So change the '#'s in the following command to the appropriate numbers:
sudo umount /Volumes/NAME_OF_YOUR_WINDOWS_DRIVE
sudo bless -device /dev/disk#s# --legacy --setBoot
#You could use -folder but this assumes the drive is always mounted and writeable
#Then a slightly cleaner restart than shutdown -r (which sends a kill message)
osascript -e 'delay 1
tell application "System Events" to restart' &
You can save that as part of a shell script or use Platypus to create an actual runnable application that will run the shell script for you.
Additionally there are other options listed on MacOSXHints