Malware
Malware is short for malicious software: software
designed specifically to damage or disrupt a system,
such as a virus, worm, or a Trojan horse.
Many times, users do not know that spyware, adware,
or malware is even installed on their computer.
Symptoms and characteristics of systems that are
infected with this software include, but are not
limited to:

slower
than usual Internet connection and performance

slower
overall performance of the computer itself

increased
amount of popup advertisements; this is especially
noticeable when a popup blocker is installed and
running.