Oh, and just in case, backup any data important data on the drive you'll be resizing. You know, just in case something happens.
BTW, if you'd like, choose expert install so you're sure your main drive isn't formatted. if you dont want to just dont mess with the main Windows partition. You'll see it. It'll be dark blue and when you click on it it'll say 2gb. Dont touch it!!! And just icase a dialog does come up asking you to format
hda (I think thats the default name for the main partition) click no.