Spyware and Pop Up Software
Spyware - There is many products and websites on the internet that tries to get you to download a file from their site. The file may be safe to download but the website also installs other programs or files on your computer. Many of them are able to do it legally because of a legal notice (better known as the fine print that majority of people doesn't read) that is listed on the page where you downloaded the file. By downloading the file or program you are giving them permission to install the other program. Some spyware will not harm your system but others can.
Pop up software - This software is good in its nature because it stops the unwanted pop ups. However, they are in competition with the companies that makes the pop ups. The people that makes the pop ups changes the way it works and therefore the pop up software has to make updates to their software. This can lead to software corruption or conflicts. I hate pop ups myself but until they are outlawed and stopped, I am stuck with them. I don't recommend the pop up software because of the number of changes that takes place. If you do use the software, I strongly suggest keeping a backup of your important data.