The most familiar and most widely adopted monitoring tool is the cookie. A cookie is a small data file - usually containing a unique identification number - that your Web browser stores on your hard drive when a site sends it a certain command. Every time you request a page or item from the site, the server connects that request to your unique identification number, giving the company an exact record of what you viewed on its site.
    Some Web sites use the cookies to keep track of an online "shopping cart"; others want to record every click you make at a site. Cookies aren't necessarily malicious, but they do let sites or advertisers make a click-by-click record of your surfing habits. Cookies from ubiquitous ad servers like DoubleClick can track you on every site for which they serve ads.

Sites use cookies for various reasons. If you completely disable them, many Web sites won't work the way you want them to. But you can limit the number of cookies your browser consumes.
    Use cookie blockers: The free Cookiewall at Analogx.com and Guidescope at Guidescope.com , for example, let you set cookie rules for each site you visit.
    Periodically clear your cookie file: If you use Netscape, search for a file named cookies.txt and delete it. If you use Internet Explorer, navigate to C:\Windows\Temporary Internet Files and C:\Windows\ Cookies and delete the entire contents of both files.

Insidious and more difficult to fight than cookies, Web Bugs are minute, invisible graphics that load with a Web page. Because Web Bugs behave just like banner ads, they provide your computer's unique IP address and the location of the page you're looking at to the server that sends the bugs - but because you don't see them, you never know they're there. Web Bugs can also identify you by setting cookies and, if you return to the same page later, retrieving them.

Web Bugs are basically graphics files. If you disable image loading in your browser, you won't be bugged, but you won't see any other graphics either, - and that's a trade-off most people aren't willing to make. But these tricky files can be fooled.
    Stop cookies first: Before you try to block the Web Bug itself from loading, you have to disable cookies. You can use the tools listed in the column at the left. If you use Internet Explorer 5.5, you can use a beta tool that lets you block or allow cookies on a page-by-page basis. Download it from microsoft.com/ windows/ ie/ download/ preview/ privacy.htm.
    Use privacy-protecting Web proxy software: Guidescope, Freedom from zeroknowledge.com, and Internet Junkbuster from junkbusters.com are all forms of proxy software. They block your real IP address by loading the Web page first to their servers and then to your computer. The drawback: Using Web proxies can slow your surfing.

Trojan Horses present themselves to the victim as something worth possessing - such as a small game or an image file - to mask their true mission: to sneak into a PC and surreptitiously monitor, control, damage, or steal data. Trojan horses themselves may not cause damage, but they let hackers or spies sift through your PC's files, disable virus-checking software, or even use your PC to mount a distributed denial of service attack against another PC on the Internet. Legitimate sites typically do not use Trojan horses.

    Use antivirus software: Most tools can detect nearly all Trojan horses. To protect against emerging new ones, keep your virus definition files updated.
    Use a Trojan horse remover: Some antivirus tools detect but can't remove Trojan horses. Simply Super Softwares's Trojan Revover at simplysup.com is designed to rid your PC of sneak attackers.
   Maintain a personal firewall: A firewall can protect you even if a Trojan horse gets in. Software like ZoneAlarm, which is free from zonelabs.com, can tell you if a program surreptitiously lets someone else view or control your PC's data.
    Don't open attachments: Most Trojan horses arrive as email attachments, so never open an .exe file until you know exactly what it does. Then scan the file with antivirus software anyway, before you open and run it.