Boot Camp is on your iMac hard drive, in Applications, then Utilities. Look for 'Boot Camp Assistant'
When you run the Boot Camp utility, and your Windows partition already exists, Boot Camp will ask you if you want to remove that partition. The only way that will work is if that Windows partition was originally created by the Boot Camp utility.
If no Windows partition exists, Boot Camp will ask if you want to create one.
If you get confused by the Startup Disk pref pane - there's not a list, but there are icons for each bootable partition. Just click on the one you want to choose as the Startup partition/disk