Misbehaving Programs
Sometimes when we install a program things just don't work as we expect. The program may not be as advertised or maybe we just expected something different.
Whatever the reason, we just want it gone. So we uninstall it, but the program's uninstaller leaves stuff behind that makes our computer act differently. It may give an error message saying that some file cannot be found when the computer is re-booted or started. And sometimes malware could 'hitch-a ride' with the errant program, causing an unexplained slowing of the computer's performance.
One way to stop those behaviors is to use a program like Revo Uninstaller. The free version works just fine and it removes nearly everything an errant program may leave behind. A scan of the downloaded program with your security scanner before installing should take care of the hitchhikers
There is another type of bad behavior folks have to watch for these days. Some programs want you to install stuff you don't want and make it appear you have to install it in order for the program to work. This type of misbehavior is usually attached to free programs. The program itself may be good, but not the stuff it wants to bring with it.
This unwanted stuff is usually a toolbar of some sort, or it may want you to change your browser's home page. It might want you to change your default search engine. Sometimes, though it is another program.
You might think that only disreputable programs will do this, but you would be wrong. I guess these folks want to pay their bills by getting a third party to foot the bills. In order for that to work, the unwanted stuff comes with the downloads.
To avoid the junk stuff you don't want, watch carefully when installing a free program. Read the EULA that is usually in the installed and uncheck anything you don't want. Most of the time the 'extra' stuff has a check box that is already checked 'for your convenience'. Just uncheck it. But beware, some of these extras may install even if you do uncheck those sneaky boxes.
The way things work today may not be how they will work tomorrow, so be careful when installing those 'free' programs as in reality they may come with an unwanted price.