Do you want to keep your Boot Camp partition or not? If not, just go into boot camp assistant and restore the HD. Then, reinstall Windows under Fusion using the same disc as you did before, and voila.
If you want to keep Boot Camp but just have a second VM, then just go into boot camp and create a second VM. You can name it whatever you like and it will show up as a second VM in the library. However, you will need to reinstall Windows again (not sure what the Windows EULA license ramifications are if you use the same Windows disc for the second installation). As a result, you'll have two copies of Windows on your HD, one of each partition with a corresponding hit to overall HD space, but that can be modest depending on how large you make your virtual HD on the VM. I do know for sure that if you delete the BC partition you can use the same Windows disc for the new VM with no net loss of HD space.
You can also delete the existing VM from within Fusion by deleting the files (although when I did that, I could not delete it from the library list for some reason).
Cheers