Very briefly, there are multiple varieties of software that can end up on you computer which aren't actually virus's but which are not necessarily in your interests.

At the "legitimate" end are items of software provided as part of "advert supported software". If you download a particular piece of legitimate software, the authors might not charge you for the software if you consent to be shown their adverts on a regular basis.

At the "nasty" end are bits of software that you can acquire from web sites that you may or may not know you've got (Even techies like me might not notice their arrival).

Why should you care? You might be happy with some of this software (eg willing to have a piece of free software in exchange for some free adverts), but some of this software is downright nasty:

a. Some software can change your internet access to dial premium-rate telephone numbers (£1.50/minute!) without you knowing... until you get a phone bill for hundreds of pounds,
b. Some software can track the sites you visit on the internet with all the privacy issues this brings,
c. Some software can steal your passwords, credit card details, bank details and send them to organised crime gangs to steal from you,
d. Some can use your computer to take part in massed attacks against other computers on the internet or indeed against the internet itself,
e. Some can use your computer to store files for distribution without your knowledge, such as child pornography.

My advice is... check regularly for this kind of softwrae using specialist software (anti-virus software doesn't cover this kind of attack).

We recommend 2 pieces of software-

Spybot Search & Destroy from http://www.safer-networking.org/en/ or you can download it directly. Ensure that you update it, innoculate your system and then run a full scan.

We also recommend that you download a copy of ad-aware (by a company called Lavasoft) from here. Download it and install it. Start it, and click "Check for Updates Now" to ensure it's up to date before running it.