Damn i tried that a few months ago and i screwed it up. I had to format my entire hard drive from scratch. Personally i wouldn't recommend anyone use partioning as it is a very bad way of doing things. All partitions will fail at some point (partitions will begin to fail) so in my opinion you are just asking for trouble in the long run. Best to have a dedicated harddrive for each os if you can. An external harddrive is ideal.
One of the mistakes i made was not using a fresh install of windows7. Mine was on there for about a year so maybe that was too long for me to use so i would definitely recommend a fresh install of windows7 befoe you do anything.
Probably the best thing you can do is try using AROS on a memory stick instead of using a partition. This will keep things clean on your harddrive and give you time to decide if you like it or not for a more permanent solution (you said you have never used it before). Good luck and try not to screw it up like i did.
