Today I had probably my first run in with a dialer. These are programs which either ask to be installed as an ActiveX control, or just forcibly install themselves using flaws in Internet Explorer. Once installed, they silence your modem's speaker, disconnect you and then redial to another number, which is usually premium rate and often offshore too. Often, the first someone knows about them is when their phone bill arrives at the end of the month.
This happened on a housemate's computer - I've managed to avoid them myself because I've always been security-conscious, and having not used IE as my main browser for the best part of two years must have helped too.
The scary thing is that I'm unsure what specifically triggered it - I fired up IE and then navigated to Windows Update, and then it appeared. KaZaA Lite K++ was running in the background. The machine had all but the most recent critical updates (automatic updates had been turned on) and was only lacking a few 'recommended' and driver updates. In any case, I'll be running the 'usual suspects' (Ad-Aware, Spybot S&D and latterly SpywareBlaster) over it.
Update: According to ICSTIS, the UK premium rate watchdog, the company running that number is under investigation for possible breaking of the industry code of conduct. Hmmm.
What was worse is that when you clicked No, it popped up a JavaScript alert that told you to click 'Yes', then reopened the dialog. There was no way to get rid of the dialog short of closing IE or clicking 'Yes' and installing the program. Thankfully, Windows XP SP2 fixes this. And in any case, the modem isn't attached to a phone line other than through an ADSL modem so it wouldn't have made any difference.
On a related note, if any of you are running Windows XP, please turn on Automatic Updates. Service Pack 2 is coming soon (finally!) and by switching this on you will be guaranteed to get it when it comes out.