What is spyware and adware?
Spyware is software that gathers information about your web surfing habits and computer usage and reports it back to a third party.
Adware is software that pops up advertisements. Some software only pops ads when you're surfing the 'net, making it difficult to determine if there is adware installed, or it is just that website.
Often, spyware and adware is poorly written. It can cause problems with your network connection, slow down your computer, cause browser crashes, and much more.
Where does it come from?
Many peer-to-peer music sharing programs install spyware and adware. KaZaA, iMesh, Bearshare and many more P2P programs install spyware.
Another way to get it is to click "Yes" on windows asking for your permission to install software.
However, there are many legitimate programs that are installed this way, including Macromedia Flash and Microsoft Windows Update. If you're not sure if you need it, say "No".
How do I get rid of it?
There are many good tools that are freely available.
If you are having problems with spyware or adware, installing and running all four programs listed above will fix most problems.