You must have seen some sites often asking the users to install some plugins on their pc in order to view or use their sites, these plugins might be some malicious spywares which are actually third party traffic trackers and these automated downloader/uploader agents continuously transfer updates to third party companies which track user visit ratings to respective sites(they keep track on sites/web urls every user visits) and moreover they also keep on consuming your internet usage credit(if any limits apply), so before you install any plugins make sure you trust the site.
Common symptoms(traceable by common users)
1. The most common symptom of this is reduced browsing speed(as some speed is used up by these spywares while transacting with the trackers)
2. You can also figure out that your pc has been infected by a spyware when you see random sign up pages from unknown sites opening up on your pc without any effort made by you to open them, it usually happens while opening any sites that you visit often or you have just started visiting often for some days/weeks.
It is suggested to immediately scan your pc with antispyware and antivirus softwares.
as soon as you locate the infection, it is good to first disconnect your internet and then delete the infection at the moment and restart your pc.
Posted by anant01 
anant01