Spyware programs.
Spywares are some kind of malicious programs that are purposefully developed and spread thought the internet to collect personal information of browsing habit of internet user. Spyware collect the record of the web site that you visited stored them and send back to the creator of spyware. The data they collected can be use directly by the developer or being sale to third party or other business. Spyware are heavily use to devise targeted marketing campaign.
Spyware obviously use your computer resources and bandwidth that make your computer running slower. To face such situation you have to equip your computer with anti spyware program. There are many anti spyware programs out there that you can use. However, it is advisable for you to buy anti spyware program as they are more powerful than free versions anti spyware program.