Paranoid steps
1) Save a current image of the OS just with WD and your complete applications and data (by using Macrium for example).
2) Install antivirus you want to use.
3) At the expiration of the license, or when you want to change antivirus, you can restore the previous clean image.
4) Install a new antivirus and so on.
Absolutely no antivirus leftovers and on my system the restore of an image requires just 10 minutes.