i guess this depends on what "easy" means really. often the easiest to get something up and running is not the easiest thing to manage in the long run. *MY* preference is to do a little groundwork in the first place to benefit in the long run. in your situation, i would say remove the linux drive, put windows in and install like a normal dedicated windows system, new drive runnign as master. then put the linux drive back in as master, and the windows drive as slave. then boot into linux exactly as you always have before. then you just need to add a grub entry to the linux system to boot to the windows one. this is where the little trick comes in in that you need to make windows *think* that it is the master drive when it boots, so we tell the bios to lie via grub: http://www.gnu.org/software/grub/manual/grub.html#map
basically add a new section at the bottom of the /boot/grub/grub.conf file that says something liek
map (hd0) (hd1)
map (hd1) (hd0)
this says, make the master the slave, the slave the master, use the real slave drive without looking for linux there and use it as a valid boot device. that'll then appear in the boot menu next time and off you go. the main benefit here is that windows doesn't have a clue at all that the linux drive exists. normally you'd end up having windows as the master drive and have grub on that drive too, which is really horrible, especially with Windows affinity to overwrite bootsectors without asking you. this way windwos can not damage anything and if the linux drive goes tits up, just remove it, put windows back as the real master drive and it'll still boot without a second thought.