1. It is not designed to harm your PC. It usually is just what the term says. It spies on you. It does so by creating some kind of an interface for someone else apart from you to monitor the activity of your computer. It normally records the Websites that you visit and your browsing activity. This information is used by Website owners to send you spam mails and pop-up advertisements.
2. Spyware is intrusive. It is an invasion of your privacy. Spyware generally has programs that record your keystrokes, can your hard disk and web browser settings. The maker of such programs stems this data which they sell to a third party as information for market research. Sometimes this data can be misused by criminals.
1. This is not covert software. This is a more acceptable form of freeware. These are advertisements that are bundled with free programs. When these free programs are downloaded by you.
These days Adware is also not so harmless.
Adware now comes with software which is similar to spyware and records your browsing activity. If your computer is slowing down, sometimes freezing altogether, there is a great likelihood that you have adware and spyware on the computer. Some of the obvious signs of spyware infection is that you are getting pop up ads that are not related to the Website that you are browsing.
2. Spyware pop-ups commonly are advertisements linked with porn sites. Moreover, if your computer is slow and takes time to boot up, it is best to get it cleaned. It is infected with spyware.
1. Viruses are malicious. They do not relate with Marketing, or are not designed to spy on you. They are simply created to cause your computer harm. They will enter your computer, wreak havoc and then initiate self replication and go and infect other computers in your network. Some of them are so deadly that they not only destroy your data, they even corrupt your hard disk and infect as many other computers as they can.
There are many antivirus programs available these days that provide anti spyware and anti adware utilities. They can scan your computer for all three infections, and even remove them.
Some programs focus primarily on virus protection, others on spyware and adware protection and in these days it is advisable not to go for them. Combination software works the best.
Combination software scans your computer, removes all the components of virus and adware from the system registry and program files. It is always good to get yourself a good quality protection which regularly updates the virus and spyware definitions.
Do not download from suspicious sites and do not get misled into thinking that the nifty program that you are downloading contains only adware. More often than not, these adware are actually spyware just waiting to set down your system and harming you. Always use a browser which has a good firewall. This is good for your PC's health.