Spyware and Malware
Quite possibly the most unethical, most annoying and biggest pain in the butt problem for a computer user is spyware and malware. The stupid little creatures that create and distribute this stuff are some of the lowest life forms that have access to the internet.
They have tools which install themselves in a number of ways, often over web sites and sometimes embedded in other software. many times it's fivilous little toy installs that appeal to kids such as smillies for IM's, font installs, screen saver and desktop theme installs and things of that nature. These apps or services then can do any number of things from watch what you do and where you go on the net, reporting back to their creator, then issuing popup ads to your PC for appropriate material, or just for anything.
Some great free stuff is out there. Lavasoft has the free and fabulous Adaware which scans for known spyware and malware in your system and removes it, and they also sell a pro version. It's definitions are updated regularly and it is very effective. Another great application is Hijack This! which finds and removes browser hijacking apps and scripts. One more I will mention deals specifically with one of the most aggressive of malware manufacturers and distributors, Cool Web Search. CWShredder is an application that deals specifically with this group's software, they tend to be very aggressive and very sneaky in the ways the stop users from removing their applications. If you get infected with a CWS application, you are in for quite a battle.
There are many others so look around the internet.
A friend of mine runs the web site that is widely considered the number one resource for spyware info, anti-spyware tools and more. It is now his full-time job and mission to find and prosecute these losers. The web site is SpyWareInfo.com. At this web site you will find all the info you need to keep you PC clean, with a huge message forum, regular newsletters and updates about new threats.
I highly recommend everybody go there for their spyware info rather than spew the same info here. The owner and his staff has been dealing with and fighting this crap for years and have a wealth of knowledge I couldn't even begin to compare with.