or at the very least, thrust unwanted spammy adverts in front of you.
If you find random pop up adverts appearing, when you visit otherwise reputable web sites, it is possible that your PC has picked up an infection. The sort of code that can place unwanted software and triggers on your system can also be just as equally abused to monitor what you are doing on your PC, for how long, and how often. The most innocuous use for such data is to sell to marketing companies, who want to know who and how they should target their campaigns at. The worst case scenario is that everything you type into a browser is being recorded, including personal and private financial details!
One prime example of the ad ware infestations possible I experienced was back in the UK, with a Russian lady called Tanya. Her PC, when I first inspected it, was so slow compared to its original speed, and with so many pop ups appearing, EVERY time she used the internet, the PC itself was almost unusable.
In the end I did manage to manually remove all the infections, but this was done manually, and took many, many hours over several visits. Whilst it was nice to meet the lady and her children, it would have been better to spend my time with her socially, rather than just swearing (quietly when the children were around) at the scum that write and propagate the adware out there.

Spy Sweeper is one quicker solution to such problems. The software is able to detect, and remove all sorts of nasties lurking on your PC, and at a very low cost. However, as the defences available improve, the writers of spy and adware change their methods, trying to out wit the the anti-spyware companies, so Spy Sweeper also includes FREE updates to their software, to make sure you always receive maximum protection, detection and dis-infection capabilities.