What they do to your PC?

Adware programs are a serious nuisance to be aware of and it is associated with programs like 'GAIN' which install pop-ups (not the normal webpage pop up), these are ads run by the programs installed by adware. They are also meant to monitor user's web usage and present you ads relevant to your web browsing.

Spyware are programs like keyloggers, activity monitoring software, and web site loggers. These programs can come into a machine as part of installing a new program or as a software virus. These malwares are often installed without users consent or the consent is obtained without clearly stating about the intent.

Spywares set your home page to their desired sites. They can sit in your PC and automatically dial some telephone numbers using your PC's modem charging you by calls made.

Trojan Horse: According to legend, the Greeks won the Trojan war by hiding in a huge, hollow wooden horse to sneak into the fortified city of Troy.

The following (incomplete) list of spyware programs classifies them by their effects:

Creating pop-ups: 180 Solutions | DirectRevenue | lop (advertising, pop ups, security risk, tries to dial out at random)

Creating pop-ups, damaging and/or slowing computers: Bonzi Buddy | Cydoor | Gator, made by the Claria Corporation (Advertising, pop ups, privacy violation, significant security risk, partially disables firewalls, some stability issues. Known for its difficulty in removing once installed.). New.net (security risk, stability issues, common cause of inability to connect) | ShopAtHomeSelect

Hijacking browsers: CoolWebSearch - most well-known browser hijacker | Euniverse | Xupiter

Committing fraud: XXXDial

Stealing information: Back Orifice (arguably better categorized as a Trojan Horse, since its open source code militates against secrecy and -- unlike most spyware -- it has no commercial motive. | VX2

Masquerading as a spyware-remover: Spyware Nuker

Miscellaneous: Internet Optimizer (Advertising, fake alert messages, possible privacy violation, security risk), MarketScore (Claims to speed up Internet connections: serious privacy violation, loss of Internet connection on some systems), CnsMin (Made in China; privacy violation. Preset in many Japanese PCs as JWord!)