The quirky thing about Spyware is that you wouldn't know the repercussions of playing host to Spyware until you know what it can do to you. Since you are better off any day without this, the first thing to do, as an effort to eliminate spyware totally from the system, is to know how it finds its way in. How does spyware enter a PC?
Web Browsers: Web Browsers are the breeding grounds for spyware. Usually taking presence as an Active X control, A browser plug-in, a browser helper object or just a simple application ( a stand-alone executable), these applications find their way in when the security systems on your computer just don't do their job well ( or worse: you never had an anti-virus installed In the first place); permitting Active X controls to run without any caution.
E-mail: As the official business messenger, email carries tons of payload that could reek of spyware and leave a trail of destruction in its wake. HTML emails are subjected to the same vulnerabilities that cause spyware to be active on web sites by showing an email message which is intended to make users click on links they ought not to. There is a reason why you are asked if you want the images accompanying your email to be displayed because this is enough to spyware into your system, especially if you use the Outlook express since it uses some vulnerable DLLs that are employed for the Internet Explorer.
Spyware from software: Most of the software downloads that you might do has to be taken with a pinch of salt. Even legitimate software that you had paid for and then downloaded would have some spyware that could be injected into your system and you wouldn't have a clue about its existence. If you have read a software's EULA ( End User License Agreement) you would see the danger etched on the walls as “some other software might be downloaded along with the main executable of your purchase..." and the software provider can now walk away Scot-free. Now you know that ignoring fine print can get you some spyware, don't you?
Peer-to-Peer file sharing downloads: This is asking for trouble. At least this could be avoided, unlike email ( which most businesses and individuals cannot do without) and web browsers ( which almost no one can ever do without). You can certainly do without the peer-to-peer download managers like Ares and the others. These are mass breeding grounds for all sorts of digital vermin. Apart from the fact that you are downloading files illegally, you are guaranteeing virus, malware, spyware and other attacks on your system. You would open your system wide open for anyone to attack mercilessly.