PC is susceptible to outside attack by individuals who want to keep your data or cause other trouble. Adware is software which can be type like malware or run safely on your system. In any case you will want to destroy all forms of foreign software to keep your system safe.

Threats
PC is most susceptible to spyware and adware due to vulnerabilities in OS with the IE browser. MAny forms of malicious software are installed with the user's consent; however, these programs sometimes attached other programs downloaded by the user.

Effects
Spyware and adware make your system slow, intense resources and collecting personal data on the sites that you visit. Some programs record keystrokes, stealing your passwords, credit card numbers and other secret info.

Significance
Some malware programs run safely in the background while others seem to take over your system. If ads pop up some times that is one symptom that your system is infected

Solutions
Keep your antivirus install antivirus programs like Spybot Search & Destroy or Spyware Doctor.

Warning
Always run your antivirus program when downloading any new software as well as spyware and adware removers.