If you're using Windows, you may very well have spyware on your computer. This was likely installed without your knowledge and consent.
Common signs that you have spyware:
You don't want spyware on your computer. Why?
First, the sole purpose of most spyware is to track your actions on the Internet and deliver advertisements. The tracking of your personal information is often not limited to which sites you visit. It will also happily collect names, addresses, e-mail addresses, etc. This information is often sold to anyone who will pay for it. I don't know why you would want this either, but apparently some companies have based their entire business model around it. Bleh desu...
Second, spyware can slow down and crash your computer.
Third, most spyware has the ability to download and install software without your knowledge and permission. This action, helpful to nobody but the advertiser, can also bring in more spyware and viruses/Trojan Horse programs. Yuck! Fourth... and possibly worst, some spyware has the ability to reconfigure dialup connection settings to cause you to connect to 900 numbers and generate truly massive phone charges. (Typical behavior of 'porn dialers').
So how do you go about getting rid of spyware?
There are several easy to use automatic products out there that are worth trying.
Sites with more information:
doxdesk.com - LOTS of info on specific spyware programs
Okay, I'm done with this nonsense.